sequencer: replace error() with BUG() in update_squash_messages ()

In sequencer.c, caller only pass TODO_SQUASH or TODO_FIXUP to
update_squash_messages(), any other command passed in should be
considered as BUG. Replace `return error('unknown command')`
with `BUG('not a FIXUP or SQUASH')`.

Signed-off-by: Lidong Yan <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: brandb97

sequencer.c

@@ -2067,6 +2067,9 @@ static int update_squash_messages(struct repository *r,
 	const char *message, *body;
 	const char *encoding = get_commit_output_encoding();
 
+	if (!is_fixup(command))
+		BUG("not a FIXUP or SQUASH %d", command);
+
 	if (ctx->current_fixup_count > 0) {
 		struct strbuf header = STRBUF_INIT;
 		char *eol;
@@ -2134,8 +2137,7 @@ static int update_squash_messages(struct repository *r,
 		strbuf_addstr(&buf, "\n\n");
 		strbuf_add_commented_lines(&buf, body, strlen(body),
 					   comment_line_str);
-	} else
-		return error(_("unknown command: %d"), command);
+	}
 	repo_unuse_commit_buffer(r, commit, message);
 
 	if (!res)