sequencer: assign only free()able strings to gpg_sign

The gpg_sign member of the replay_opts structure is of type `char *`,
meaning that the sequencer deems the string to which gpg_sign points to
be under its custody, i.e. it needs to be free()d by the sequencer.

Therefore, let's only assign malloc()ed buffers to it.

Reported-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -160,7 +160,7 @@ static int git_sequencer_config(const char *k, const char *v, void *cb)
 	}
 
 	if (!strcmp(k, "commit.gpgsign")) {
-		opts->gpg_sign = git_config_bool(k, v) ? "" : NULL;
+		opts->gpg_sign = git_config_bool(k, v) ? xstrdup("") : NULL;
 		return 0;
 	}
 

t/t3404-rebase-interactive.sh

@@ -1318,6 +1318,16 @@ test_expect_success 'editor saves as CR/LF' '
 
 SQ="'"
 test_expect_success 'rebase -i --gpg-sign=<key-id>' '
+	test_when_finished "test_might_fail git rebase --abort" &&
+	set_fake_editor &&
+	FAKE_LINES="edit 1" git rebase -i --gpg-sign="\"S I Gner\"" HEAD^ \
+		>out 2>err &&
+	test_i18ngrep "$SQ-S\"S I Gner\"$SQ" err
+'
+
+test_expect_success 'rebase -i --gpg-sign=<key-id> overrides commit.gpgSign' '
+	test_when_finished "test_might_fail git rebase --abort" &&
+	test_config commit.gpgsign true &&
 	set_fake_editor &&
 	FAKE_LINES="edit 1" git rebase -i --gpg-sign="\"S I Gner\"" HEAD^ \
 		>out 2>err &&