sequencer: support amending commits

This teaches the run_git_commit() function to take an argument that will
allow us to implement "todo" commands that need to amend the commit
messages ("fixup", "squash" and "reword").

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

sequencer.c

@@ -484,7 +484,7 @@ static char **read_author_script(void)
  * author metadata.
  */
 static int run_git_commit(const char *defmsg, struct replay_opts *opts,
-			  int allow_empty, int edit)
+			  int allow_empty, int edit, int amend)
 {
 	char **env = NULL;
 	struct argv_array array;
@@ -513,6 +513,8 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts,
 	argv_array_push(&array, "commit");
 	argv_array_push(&array, "-n");
 
+	if (amend)
+		argv_array_push(&array, "--amend");
 	if (opts->gpg_sign)
 		argv_array_pushf(&array, "-S%s", opts->gpg_sign);
 	if (opts->signoff)
@@ -786,7 +788,7 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 	}
 	if (!opts->no_commit)
 		res = run_git_commit(opts->edit ? NULL : git_path_merge_msg(),
-				     opts, allow, opts->edit);
+				     opts, allow, opts->edit, 0);
 
 leave:
 	free_message(commit, &msg);