dump_marks(): remove a redundant call to rollback_lock_file()

When commit_lock_file() fails, it now always calls
rollback_lock_file() internally, so there is no need to call that
function here.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1832,10 +1832,8 @@ static void dump_marks(void)
 	}
 
 	if (commit_lock_file(&mark_lock)) {
-		int saved_errno = errno;
-		rollback_lock_file(&mark_lock);
 		failure |= error("Unable to commit marks file %s: %s",
-			export_marks_file, strerror(saved_errno));
+			export_marks_file, strerror(errno));
 		return;
 	}
 }