show-branch: Fix another performance problem.

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>