t3404: add a test for the --gpg-sign option

For the upcoming rebase--helper work (which will accelerate the
interactive rebase noticably), it is important to verify that the
--gpg-sign option is handled properly.

Please note that this patch does this on the cheap, by verifying that
the expected option is printed in the message of the 'edit' operation.

We really should test that the interactive rebase signs the commits
properly, iff GPG is available. This test is left for later.

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

t/t3404-rebase-interactive.sh

@@ -1281,4 +1281,12 @@ test_expect_success 'editor saves as CR/LF' '
 	)
 '
 
+SQ="'"
+test_expect_success 'rebase -i --gpg-sign=<key-id>' '
+	set_fake_editor &&
+	FAKE_LINES="edit 1" git rebase -i --gpg-sign="\"S I Gner\"" HEAD^ \
+		>out 2>err &&
+	grep "$SQ-S\"S I Gner\"$SQ" err
+'
+
 test_done