show-branch: Fix another performance problem.

When naming commits, stop walking the parent chain as soon as we find
a commit that already has a name. The parent chain of that commit will
be walked later on in any case (or may even have been walked already).
This avoids O(n^2) behavior; on a tree where show-branch displays 6800
commits, the total run time drops from 77 seconds to 5 seconds.

Signed-off-by: Alexandre Julliard <julliard@winehq.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-show-branch.c

@@ -89,6 +89,8 @@ static int name_first_parent_chain(struct commit *c)
 			name_parent(c, p);
 			i++;
 		}
+		else
+			break;
 		c = p;
 	}
 	return i;