Remove hash in git-describe in favor of util slot.

Remove hash in git-describe in favor of util slot.

Currently we don't use the util field of struct commit but we want
fast access to the highest priority name that references any given
commit object during our matching loop.  A really simple approach
is to just store the name directly in the util field.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>