[PATCH] Making it easier to find which change introduced a bug

@@ -62,7 +62,7 @@ SCRIPTS=git git-apply-patch-script git-m
 	git-format-patch-script git-sh-setup-script git-push-script \
 	git-branch-script git-parse-remote git-verify-tag-script \
 	git-ls-remote-script git-clone-dumb-http git-rename-script \
-	git-request-pull-script
+	git-request-pull-script git-bisect-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 \

@@ -0,0 +1,158 @@
+#!/bin/sh
+. git-sh-setup-script || dir "Not a git archive"
+
+usage() {
+    echo >&2 'usage: git bisect [start | bad | good | next | reset]
+git bisect start		reset bisect state and start bisection.
+git bisect bad [<rev>]		mark <rev> a known-bad revision.
+git bisect good [<rev>...]	mark <rev>... known-good revisions.
+git bisect next			find next bisection to test and check it out.
+git bisect reset [<branch>]	finish bisection search and go back to branch.'
+    exit 1
+}
+
+bisect_autostart() {
+	test -d "$GIT_DIR/refs/bisect" || {
+		echo >&2 'You need to start by "git bisect start"'
+		if test -t 0
+		then
+			echo >&2 -n 'Do you want me to do it for you [Y/n]? '
+			read yesno
+			case "$yesno" in
+			[Nn]*)
+				exit ;;
+			esac
+			bisect_start
+		else
+			exit 1
+		fi
+	}
+}
+
+bisect_start() {
+        case "$#" in 0) ;; *) usage ;; esac
+	#
+	# Verify HEAD. If we were bisecting before this, reset to the
+	# top-of-line master first!
+	#
+	head=$(readlink $GIT_DIR/HEAD) || die "Bad HEAD - I need a symlink"
+	case "$head" in
+	refs/heads/bisect*)
+		git checkout master || exit
+		;;
+	refs/heads/*)
+		;;
+	*)
+		die "Bad HEAD - strange symlink"
+		;;
+	esac
+
+	#
+	# Get rid of any old bisect state
+	#
+	rm -f "$GIT_DIR/refs/heads/bisect"
+	rm -rf "$GIT_DIR/refs/bisect/"
+	mkdir "$GIT_DIR/refs/bisect"
+}
+
+bisect_bad() {
+	bisect_autostart
+        case "$#" in 0 | 1) ;; *) usage ;; esac
+	rev=$(git-rev-parse --revs-only --verify --default HEAD "$@") || exit
+	echo "$rev" > "$GIT_DIR/refs/bisect/bad"
+	bisect_auto_next
+}
+
+bisect_good() {
+	bisect_autostart
+        case "$#" in
+	0)    revs=$(git-rev-parse --verify HEAD) || exit ;;
+	*)    revs=$(git-rev-parse --revs-only "$@") || exit ;;
+	esac
+	for rev in $revs
+	do
+	    echo "$rev" >"$GIT_DIR/refs/bisect/good-$rev"
+	done
+	bisect_auto_next
+}
+
+bisect_next_check() {
+	next_ok=no
+        test -f "$GIT_DIR/refs/bisect/bad" &&
+	case "$(cd "$GIT_DIR" && echo refs/bisect/good-*)" in
+	refs/bisect/good-\*) ;;
+	*) next_ok=yes ;;
+	esac
+	case "$next_ok,$1" in
+	no,) false ;;
+	no,fail)
+	    echo >&2 'You need to give me at least one good and one bad revisions.'
+	    exit 1 ;;
+	*)
+	    true ;;
+	esac
+}
+
+bisect_auto_next() {
+	bisect_next_check && bisect_next
+}
+
+bisect_next() {
+        case "$#" in 0) ;; *) usage ;; esac
+	bisect_autostart
+	bisect_next_check fail
+	bad=$(git-rev-parse --verify refs/bisect/bad) &&
+	good=$(git-rev-parse --sq --revs-only --not \
+		$(cd "$GIT_DIR" && ls refs/bisect/good-*)) &&
+	rev=$(eval "git-rev-list --bisect $good $bad") || exit
+	nr=$(eval "git-rev-list $rev $good" | wc -l) || exit
+	if [ "$nr" -le "1" ]; then
+	    echo "$bad is first bad commit"
+	    git-diff-tree --pretty $bad
+	    exit 0
+	fi
+	echo "Bisecting: $nr revisions left to test after this"
+	echo "$rev" > "$GIT_DIR/refs/heads/new-bisect"
+	git checkout new-bisect || exit
+	mv "$GIT_DIR/refs/heads/new-bisect" "$GIT_DIR/refs/heads/bisect" &&
+	ln -sf refs/heads/bisect "$GIT_DIR/HEAD"
+}
+
+bisect_reset() {
+	case "$#" in
+	0) branch=master ;;
+	1) test -f "$GIT_DIR/refs/heads/$1" || {
+	       echo >&2 "$1 does not seem to be a valid branch"
+	       exit 1
+	   }
+	   branch="$1" ;;
+        *)
+	    usage ;;
+	esac
+	git checkout "$branch" &&
+	rm -fr "$GIT_DIR/refs/bisect"
+	rm -f "$GIT_DIR/refs/reads/bisect"
+}
+
+case "$#" in
+0)
+    usage ;;
+*)
+    cmd="$1"
+    shift
+    case "$cmd" in
+    start)
+        bisect_start "$@" ;;
+    bad)
+        bisect_bad "$@" ;;
+    good)
+        bisect_good "$@" ;;
+    next)
+        # Not sure we want "next" at the UI level anymore.
+        bisect_next "$@" ;;
+    reset)
+        bisect_reset "$@" ;;
+    *)
+        usage ;;
+    esac
+esac