[PATCH] Extend "git reset" to take a reset point

This was triggered by a query by Sam Ravnborg, and extends "git reset" to
reset the index and the .git/HEAD pointer to an arbitrarily named point.

For example

	git reset HEAD^

will just reset the current HEAD to its own parent - leaving the working
directory untouched, but effectively un-doing the top-most commit. You
might want to do this if you realize after you committed that you made a
mistake that you want to fix up: reset your HEAD back to its previous
state, fix up the working directory and re-do the commit.

If you want to totally un-do the commit (and reset your working directory
to that point too), you'd first use "git reset HEAD^" to reset to the
parent, and then do a "git checkout -f" to reset the working directory
state to that point in time too.
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-reset-script

 #!/bin/sh
 . git-sh-setup-script || die "Not a git archive"
-git-read-tree --reset HEAD
+rev=$(git-rev-parse --revs-only --verify --default HEAD "$@") || exit
+rev=$(git-rev-parse --revs-only --verify $rev^0) || exit
+git-read-tree --reset "$rev" && echo "$rev" > "$GIT_DIR/HEAD"
 git-update-cache --refresh
 rm -f "$GIT_DIR/MERGE_HEAD"