Add some installation notes in INSTALL

Jens was the second person who hadn't heard of the "merge" program, and
didn't have it installed.  So document as many dependency and install
issues as I can think of.

diff --git a/INSTALL b/INSTALL
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+++ b/INSTALL
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+
+               Git installation
+
+Normally you can just do "make" followed by "make install", and that
+will install the git programs in your own ~/bin/ directory.  If you want
+to do a global install, you can do
+
+       make prefix=/usr install
+
+(or prefix=/usr/local, of course).  Some day somebody may send me a RPM
+spec file or something, and you can do "make rpm" or whatever.
+
+Issues of note:
+
+ - git normally installs a helper script wrapper called "git", which
+   conflicts with a similarly named "GNU interactive tools" program.
+
+   Tough.  Either don't use the wrapper script, or delete the old GNU
+   interactive tools.  None of the core git stuff needs the wrapper,
+   it's just a convenient shorthand and while it is documented in some
+   places, you can always replace "git commit" with "git-commit-script"
+   instead. 
+
+   But let's face it, most of us don't have GNU interactive tools, and
+   even if we had it, we wouldn't know what it does.  I don't think it
+   has been actively developed since 1997, and people have moved over to
+   graphical file managers.
+
+ - Git is reasonably self-sufficient, but does depend on a few external
+   programs and libraries:
+
+       - "zlib", the compression library. Git won't build without it.
+
+       - "openssl".  The git-rev-list program uses bignum support from
+         openssl, and unless you specify otherwise, you'll also get the
+         SHA1 library from here.
+
+         If you don't have openssl, you can use one of the SHA1 libraries
+         that come with git (git includes the one from Mozilla, and has
+         its own PowerPC-optimized one too - see the Makefile), and you
+         can avoid the bignum support by excising git-rev-list support
+         for "--merge-order" (by hand).
+
+       - "libcurl".  git-http-pull uses this.  You can disable building of
+         that program if you just want to get started. 
+
+       - "GNU patch" to generate patches.  Of course, you don't _have_ to
+         generate patches if you don't want to, but let's face it, you'll
+         be wanting to. Or why did you get git in the first place?
+
+         Non-GNU versions of the patch program don't generally support
+         the unified patch format (which is the one git uses), so you
+         really do want to get the GNU one.  Trust me, you will want to
+         do that even if it wasn't for git.  There's no point in living
+         in the dark ages any more. 
+
+       - "merge", the standard UNIX three-way merge program.  It usually
+         comes with the "rcs" package on most Linux distributions, so if
+         you have a developer install you probably have it already, but a
+         "graphical user desktop" install might have left it out.
+
+         You'll only need the merge program if you do development using
+         git, and if you only use git to track other peoples work you'll
+         never notice the lack of it.