Add dangling objects tips.

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

diff --git a/Documentation/howto/dangling-objects.txt b/Documentation/howto/dangling-objects.txt
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+From: Linus Torvalds <torvalds@linux-foundation.org>
+Subject: Re: Question about fsck-objects output
+Date: Thu, 25 Jan 2007 12:01:06 -0800 (PST)
+Message-ID: <Pine.LNX.4.64.0701251144290.25027@woody.linux-foundation.org>
+Archived-At: <http://permalink.gmane.org/gmane.comp.version-control.git/37754>
+Abstract: Linus describes what dangling objects are, when they
+ are left behind, and how to view their relationship with branch
+ heads in gitk
+
+On Thu, 25 Jan 2007, Larry Streepy wrote:
+
+> Sorry to ask such a basic question, but I can't quite decipher the output of
+> fsck-objects.  When I run it, I get this:
+>
+>  git fsck-objects
+> dangling commit 2213f6d4dd39ca8baebd0427723723e63208521b
+> dangling commit f0d4e00196bd5ee54463e9ea7a0f0e8303da767f
+> dangling blob 6a6d0b01b3e96d49a8f2c7addd4ef8c3bd1f5761
+>
+>
+> Even after a "repack -a -d" they still exist.  The man page has a short
+> explanation, but, at least for me, it wasn't fully enlightening. :-)
+>
+> The man page says that dangling commits could be "root" commits, but since my
+> repo started as a clone of another repo, I don't see how I could have any root
+> commits.  Also, the page doesn't really describe what a dangling blob is.
+>
+> So, can someone explain what these artifacts are and if they are a problem
+> that I should be worried about?
+
+The most common situation is that you've rebased a branch (or you have
+pulled from somebody else who rebased a branch, like the "pu" branch in
+the git.git archive itself).
+
+What happens is that the old head of the original branch still exists, as
+does obviously everything it pointed to. The branch pointer itself just
+doesn't, since you replaced it with another one.
+
+However, there are certainly other situations too that cause dangling
+objects. For example, the "dangling blob" situation you have tends to be
+because you did a "git add" of a file, but then, before you actually
+committed it and made it part of the bigger picture, you changed something
+else in that file and committed that *updated* thing - the old state that
+you added originally ends up not being pointed to by any commit/tree, so
+it's now a dangling blob object.
+
+Similarly, when the "recursive" merge strategy runs, and finds that there
+are criss-cross merges and thus more than one merge base (which is fairly
+unusual, but it does happen), it will generate one temporary midway tree
+(or possibly even more, if you had lots of criss-crossing merges and
+more than two merge bases) as a temporary internal merge base, and again,
+those are real objects, but the end result will not end up pointing to
+them, so they end up "dangling" in your repository.
+
+Generally, dangling objects aren't anything to worry about. They can even
+be very useful: if you screw something up, the dangling objects can be how
+you recover your old tree (say, you did a rebase, and realized that you
+really didn't want to - you can look at what dangling objects you have,
+and decide to reset your head to some old dangling state).
+
+For commits, the most useful thing to do with dangling objects tends to be
+to do a simple
+
+       gitk <dangling-commit-sha-goes-here> --not --all
+
+which means exactly what it sounds like: it says that you want to see the
+commit history that is described by the dangling commit(s), but you do NOT
+want to see the history that is described by all your branches and tags
+(which are the things you normally reach). That basically shows you in a
+nice way what the danglign commit was (and notice that it might not be
+just one commit: we only report the "tip of the line" as being dangling,
+but there might be a whole deep and complex commit history that has gotten
+dropped - rebasing will do that).
+
+For blobs and trees, you can't do the same, but you can examine them. You
+can just do
+
+       git show <dangling-blob/tree-sha-goes-here>
+
+to show what the contents of the blob were (or, for a tree, basically what
+the "ls" for that directory was), and that may give you some idea of what
+the operation was that left that dangling object.
+
+Usually, dangling blobs and trees aren't very interesting. They're almost
+always the result of either being a half-way mergebase (the blob will
+often even have the conflict markers from a merge in it, if you have had
+conflicting merges that you fixed up by hand), or simply because you
+interrupted a "git fetch" with ^C or something like that, leaving _some_
+of the new objects in the object database, but just dangling and useless.
+
+Anyway, once you are sure that you're not interested in any dangling
+state, you can just prune all unreachable objects:
+
+       git prune
+
+and they'll be gone. But you should only run "git prune" on a quiescent
+repository - it's kind of like doing a filesystem fsck recovery: you don't
+want to do that while the filesystem is mounted.
+
+(The same is true of "git-fsck-objects" itself, btw - but since
+git-fsck-objects never actually *changes* the repository, it just reports
+on what it found, git-fsck-objects itself is never "dangerous" to run.
+Running it while somebody is actually changing the repository can cause
+confusing and scary messages, but it won't actually do anything bad. In
+contrast, running "git prune" while somebody is actively changing the
+repository is a *BAD* idea).
+
+                       Linus
+