git-revert: revert an existing commit.

Given one existing commit, revert the change the patch
introduces, and record a new commit that records it.  This
requires your working tree to be clean (no modifications from
the HEAD commit).

This is based on what Linus posted to the list, with
enhancements he suggested, including the use of -M to attempt
reverting renames.

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

Makefile

@@ -70,6 +70,7 @@ SCRIPTS=git git-apply-patch-script git-merge-one-file-script git-prune-script \
 
 SCRIPTS += git-count-objects-script
 # SCRIPTS += git-send-email-script
+SCRIPTS += git-revert-script
 
 PROG=   git-update-cache git-diff-files git-init-db git-write-tree \
 	git-read-tree git-commit-tree git-cat-file git-fsck-cache \

git-revert-script

@@ -0,0 +1,37 @@
+#!/bin/sh
+. git-sh-setup-script || die "Not a git archive"
+
+# We want a clean tree and clean index to be able to revert.
+status=$(git status)
+case "$status" in
+'nothing to commit') ;;
+*)
+	echo "$status"
+	die "Your working tree is dirty; cannot revert a previous patch." ;;
+esac
+
+rev=$(git-rev-parse --no-flags --verify --revs-only "$@") &&
+commit=$(git-rev-parse --verify "$rev^0") || exit
+if git-diff-tree -R -M -p $commit | git-apply --index &&
+   msg=$(git-rev-list --pretty=oneline --max-count=1 $commit)
+then
+        {
+                echo "$msg" | sed -e '
+			s/^[^ ]* /Revert "/
+			s/$/"/'
+                echo
+                echo "This reverts $commit commit."
+                test "$rev" = "$commit" ||
+                echo "(original 'git revert' arguments: $@)"
+        } | git commit -F -
+else
+        # Now why did it fail?
+        parents=`git-cat-file commit "$commit" 2>/dev/null |
+                sed -ne '/^$/q;/^parent /p' |
+                wc -l`
+        case $parents in
+        0) die "Cannot revert the root commit nor non commit-ish." ;;
+        1) die "The patch does not apply." ;;
+        *) die "Cannot revert a merge commit." ;;
+        esac
+fi