rebase -p -i: handle "no changes" gracefully

Since commit 376ccb8cbb (rebase -i: style
fixes and minor cleanups), unchanged SHA-1s are no longer mapped via
$REWRITTEN.  But the updating phase was not prepared for the old head
not being rewritten.

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

git-rebase--interactive.sh

@@ -322,7 +322,12 @@ do_next () {
 		test -f "$DOTEST"/current-commit &&
 			current_commit=$(cat "$DOTEST"/current-commit) &&
 			git rev-parse HEAD > "$REWRITTEN"/$current_commit
-		NEWHEAD=$(cat "$REWRITTEN"/$OLDHEAD)
+		if test -f "$REWRITTEN"/$OLDHEAD
+		then
+			NEWHEAD=$(cat "$REWRITTEN"/$OLDHEAD)
+		else
+			NEWHEAD=$OLDHEAD
+		fi
 	else
 		NEWHEAD=$(git rev-parse HEAD)
 	fi &&

t/t3404-rebase-interactive.sh

@@ -184,6 +184,12 @@ test_expect_success 'retain authorship when squashing' '
 	git show HEAD | grep "^Author: Twerp Snog"
 '
 
+test_expect_success '-p handles "no changes" gracefully' '
+	HEAD=$(git rev-parse HEAD) &&
+	git rebase -i -p HEAD^ &&
+	test $HEAD = $(git rev-parse HEAD)
+'
+
 test_expect_success 'preserve merges with -p' '
 	git checkout -b to-be-preserved master^ &&
 	: > unrelated-file &&