Add [HOWTO] revert/branch/rebase.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/Documentation/howto/revert-branch-rebase.txt b/Documentation/howto/revert-branch-rebase.txt
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+From: Junio C Hamano <junkio@cox.net>
+To: git@vger.kernel.org
+Subject: [HOWTO] Reverting an existing commit
+Abstract: In this article, JC gives a small real-life example of using
+ 'git revert' command, and using a temporary branch and tag for safety
+ and easier sanity checking.
+Date: Mon, 29 Aug 2005 21:39:02 -0700
+Content-type: text/asciidoc
+Message-ID: <7voe7g3uop.fsf@assigned-by-dhcp.cox.net>
+
+Reverting an existing commit
+============================
+
+One of the changes I pulled into the 'master' branch turns out to
+break building GIT with GCC 2.95.  While they were well intentioned
+portability fixes, keeping things working with gcc-2.95 was also
+important.  Here is what I did to revert the change in the 'master'
+branch and to adjust the 'pu' branch, using core GIT tools and
+barebone Porcelain.
+
+First, prepare a throw-away branch in case I screw things up.
+
+------------------------------------------------
+$ git checkout -b revert-c99 master
+------------------------------------------------
+
+Now I am on the 'revert-c99' branch.  Let's figure out which commit to
+revert.  I happen to know that the top of the 'master' branch is a
+merge, and its second parent (i.e. foreign commit I merged from) has
+the change I would want to undo.  Further I happen to know that that
+merge introduced 5 commits or so:
+
+------------------------------------------------
+$ git show-branch --more=4 master master^2 | head
+! [master] Merge refs/heads/portable from http://www.cs.berkeley....
+ ! [master^2] Replace C99 array initializers with code.
+--
++  [master] Merge refs/heads/portable from http://www.cs.berkeley....
+++ [master^2] Replace C99 array initializers with code.
+++ [master^2~1] Replace unsetenv() and setenv() with older putenv().
+++ [master^2~2] Include sys/time.h in daemon.c.
+++ [master^2~3] Fix ?: statements.
+++ [master^2~4] Replace zero-length array decls with [].
++  [master~1] tutorial note about git branch
+------------------------------------------------
+
+The '--more=4' above means "after we reach the merge base of refs,
+show until we display four more common commits".  That last commit
+would have been where the "portable" branch was forked from the main
+git.git repository, so this would show everything on both branches
+since then.  I just limited the output to the first handful using
+'head'.
+
+Now I know 'master^2~4' (pronounce it as "find the second parent of
+the 'master', and then go four generations back following the first
+parent") is the one I would want to revert.  Since I also want to say
+why I am reverting it, the '-n' flag is given to 'git revert'.  This
+prevents it from actually making a commit, and instead 'git revert'
+leaves the commit log message it wanted to use in '.msg' file:
+
+------------------------------------------------
+$ git revert -n master^2~4
+$ cat .msg
+Revert "Replace zero-length array decls with []."
+
+This reverts 6c5f9baa3bc0d63e141e0afc23110205379905a4 commit.
+$ git diff HEAD ;# to make sure what we are reverting makes sense.
+$ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage.
+$ make clean test ;# make sure it did not cause other breakage.
+------------------------------------------------
+
+The reverted change makes sense (from reading the 'diff' output), does
+fix the problem (from 'make CC=gcc-2.95' test), and does not cause new
+breakage (from the last 'make test').  I'm ready to commit:
+
+------------------------------------------------
+$ git commit -a -s ;# read .msg into the log,
+                    # and explain why I am reverting.
+------------------------------------------------
+
+I could have screwed up in any of the above steps, but in the worst
+case I could just have done 'git checkout master' to start over.
+Fortunately I did not have to; what I have in the current branch
+'revert-c99' is what I want.  So merge that back into 'master':
+
+------------------------------------------------
+$ git checkout master
+$ git resolve master revert-c99 fast ;# this should be a fast forward
+Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c...
+ cache.h        |    8 ++++----
+ commit.c       |    2 +-
+ ls-files.c     |    2 +-
+ receive-pack.c |    2 +-
+ server-info.c  |    2 +-
+ 5 files changed, 8 insertions(+), 8 deletions(-)
+------------------------------------------------
+
+The 'fast' in the above 'git resolve' is not a magic.  I knew this
+'resolve' would result in a fast forward merge, and if not, there is
+something very wrong (so I would do 'git reset' on the 'master' branch
+and examine the situation).  When a fast forward merge is done, the
+message parameter to 'git resolve' is discarded, because no new commit
+is created.  You could have said 'junk' or 'nothing' there as well.
+
+There is no need to redo the test at this point.  We fast forwarded
+and we know 'master' matches 'revert-c99' exactly.  In fact:
+
+------------------------------------------------
+$ git diff master..revert-c99
+------------------------------------------------
+
+says nothing.
+
+Then we rebase the 'pu' branch as usual.
+
+------------------------------------------------
+$ git checkout pu
+$ git tag pu-anchor pu
+$ git rebase master
+* Applying: Redo "revert" using three-way merge machinery.
+First trying simple merge strategy to cherry-pick.
+Finished one cherry-pick.
+* Applying: Remove git-apply-patch-script.
+First trying simple merge strategy to cherry-pick.
+Simple cherry-pick fails; trying Automatic cherry-pick.
+Removing Documentation/git-apply-patch-script.txt
+Removing git-apply-patch-script
+Finished one cherry-pick.
+* Applying: Document "git cherry-pick" and "git revert"
+First trying simple merge strategy to cherry-pick.
+Finished one cherry-pick.
+* Applying: mailinfo and applymbox updates
+First trying simple merge strategy to cherry-pick.
+Finished one cherry-pick.
+* Applying: Show commits in topo order and name all commits.
+First trying simple merge strategy to cherry-pick.
+Finished one cherry-pick.
+* Applying: More documentation updates.
+First trying simple merge strategy to cherry-pick.
+Finished one cherry-pick.
+------------------------------------------------
+
+The temporary tag 'pu-anchor' is me just being careful, in case 'git
+rebase' screws up.  After this, I can do these for sanity check:
+
+------------------------------------------------
+$ git diff pu-anchor..pu ;# make sure we got the master fix.
+$ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage.
+$ make clean test ;# make sure it did not cause other breakage.
+------------------------------------------------
+
+Everything is in the good order.  I do not need the temporary branch
+nor tag anymore, so remove them:
+
+------------------------------------------------
+$ rm -f .git/refs/tags/pu-anchor .git/refs/heads/revert-c99
+------------------------------------------------
+
+It was an emergency fix, so we might as well merge it into the
+'release candidate' branch, although I expect the next release would
+be some days off:
+
+------------------------------------------------
+$ git checkout rc
+$ git pull . master
+Packing 0 objects
+Unpacking 0 objects
+
+* committish: e3a693c...       refs/heads/master from .
+Trying to merge e3a693c... into 8c1f5f0... using 10d781b...
+Committed merge 7fb9b7262a1d1e0a47bbfdcbbcf50ce0635d3f8f
+ cache.h        |    8 ++++----
+ commit.c       |    2 +-
+ ls-files.c     |    2 +-
+ receive-pack.c |    2 +-
+ server-info.c  |    2 +-
+ 5 files changed, 8 insertions(+), 8 deletions(-)
+------------------------------------------------
+
+And the final repository status looks like this:
+
+------------------------------------------------
+$ git show-branch --more=1 master pu rc
+! [master] Revert "Replace zero-length array decls with []."
+ ! [pu] git-repack-script: Add option to repack all objects.
+  * [rc] Merge refs/heads/master from .
+---
+ +  [pu] git-repack-script: Add option to repack all objects.
+ +  [pu~1] More documentation updates.
+ +  [pu~2] Show commits in topo order and name all commits.
+ +  [pu~3] mailinfo and applymbox updates
+ +  [pu~4] Document "git cherry-pick" and "git revert"
+ +  [pu~5] Remove git-apply-patch-script.
+ +  [pu~6] Redo "revert" using three-way merge machinery.
+  + [rc] Merge refs/heads/master from .
++++ [master] Revert "Replace zero-length array decls with []."
+  + [rc~1] Merge refs/heads/master from .
++++ [master~1] Merge refs/heads/portable from http://www.cs.berkeley....
+------------------------------------------------