rebase -i: avoid 'git reset' when possible

When picking commits whose parents have not changed, we do not need to
rewrite the commit.  We do not need to reset the working directory to
the parent's state, either.

Requested by Sverre Rabbelier.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -442,6 +442,30 @@ do_rest () {
 	done
 }
 
+# skip picking commits whose parents are unchanged
+skip_unnecessary_picks () {
+	fd=3
+	while read command sha1 rest
+	do
+		# fd=3 means we skip the command
+		case "$fd,$command,$(git rev-parse --verify --quiet $sha1^)" in
+		3,pick,"$ONTO"*|3,p,"$ONTO"*)
+			# pick a commit whose parent is current $ONTO -> skip
+			ONTO=$sha1
+			;;
+		3,#*|3,,*)
+			# copy comments
+			;;
+		*)
+			fd=1
+			;;
+		esac
+		echo "$command${sha1:+ }$sha1${rest:+ }$rest" >&$fd
+	done <"$TODO" >"$TODO.new" 3>>"$DONE" &&
+	mv -f "$TODO".new "$TODO" ||
+	die "Could not skip unnecessary pick commands"
+}
+
 # check if no other options are set
 is_standalone () {
 	test $# -eq 2 -a "$2" = '--' &&
@@ -746,6 +770,8 @@ EOF
 		has_action "$TODO" ||
 			die_abort "Nothing to do"
 
+		test -d "$REWRITTEN" || skip_unnecessary_picks
+
 		git update-ref ORIG_HEAD $HEAD
 		output git checkout $ONTO && do_rest
 		;;

t/t3404-rebase-interactive.sh

@@ -459,4 +459,15 @@ test_expect_success 'submodule rebase -i' '
 	FAKE_LINES="1 squash 2 3" git rebase -i A
 '
 
+test_expect_success 'avoid unnecessary reset' '
+	git checkout master &&
+	test-chmtime =123456789 file3 &&
+	git update-index --refresh &&
+	HEAD=$(git rev-parse HEAD) &&
+	git rebase -i HEAD~4 &&
+	test $HEAD = $(git rev-parse HEAD) &&
+	MTIME=$(test-chmtime -v +0 file3 | sed 's/[^0-9].*$//') &&
+	test 123456789 = $MTIME
+'
+
 test_done