"GIT_INDEX_FILE" environment variable

This checkin goes along with the previous one, and makes it easier to use all the normal git operations on temporary index files:

  Add support for a "GIT_INDEX_FILE" environment variable.
  
  We use that to specify alternative index files, which can be useful
  if you want to (for example) generate a temporary index file to do
  some specific operation that you don't want to mess with your main
  one with.
  
  It defaults to the regular ".git/index" if it hasn't been specified.

and it's particularly useful for doing things like "read a tree into a temporary index file, and write the result out". For example, say that you wanted to know what the Makefile looked like in a particular release, you could do

    GIT_INDEX_FILE=.tmp-index read-tree $release
    GIT_INDEX_FILE=.tmp-index checkout-cache --prefix=old- Makefile
    rm .tmp-index

and you're done. Your old Makefile version is now in "old-Makefile" (and this is also where it's nice that checkout-cache refuses to overwrite existing files by default: if you forgot or messed up the prefix, it's all good).

You can also use it to test merges without screwing up your old index file in case something goes wrong.

Did I already happen to mention that I think that the git model is the best model ever, and that I'm just not an incredibly good-looking hunk and becomingly modest, I'm smart too?

		Linus