sha1_name: avoid unnecessary sha1 lookup in find_unique_abbrev

sha1_name: avoid unnecessary sha1 lookup in find_unique_abbrev

An example where this happens is when doing an ls-tree on a tree that
contains a commit link. In that case, find_unique_abbrev is called
to get a non-abbreviated hex sha1, but still, a lookup is done as
to whether the sha1 is in the repository (which ends up looking for
a loose object in .git/objects), while the result of that lookup is
not used when returning a non-abbreviated hex sha1.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>