Allow short pack names to git-pack-objects --unpacked=.

Allow short pack names to git-pack-objects --unpacked=.

This allows us to pass just the file name of a pack rather than
the complete path when we want pack-objects to consider its
contents as though they were loose objects.  This can be helpful
if $GIT_OBJECT_DIRECTORY contains shell metacharacters which make
it cumbersome to pass complete paths safely in a shell script.

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