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>

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 &&