sequencer: fix memory leak if update_squash_messages() failed

In sequencer.c:update_squash_messages, `repo_logmsg_reencode` returns
either an allocated reencode string or commit buffer if no encode is
needed. To free `repo_logmsg_reencode` result, `repo_unuse_commit_buffer`
should be used. However, when encountering the error("unknown command..."),
the absence of `repo_unuse_commit_buffer` results in a memory leak. I
think we should add a `repo_unuse_commit_buffer` before return.

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

Author: brandb97

sequencer.c

@@ -2138,8 +2138,10 @@ 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
+	} else {
+		repo_unuse_commit_buffer(r, commit, message);
 		return error(_("unknown command: %d"), command);
+	}
 	repo_unuse_commit_buffer(r, commit, message);
 
 	if (!res)