Optimize sha1_object_info for loose objects, not concurrent repacks

Optimize sha1_object_info for loose objects, not concurrent repacks

When dealing with a repository with lots of loose objects, sha1_object_info
would rescan the packs directory every time an unpacked object was referenced
before finally giving up and looking for the loose object. This caused a lot
of extra unnecessary system calls during git pack-objects; the code was
rereading the entire pack directory once for each loose object file.

This patch looks for a loose object before falling back to rescanning the
pack directory, rather than the other way around.

Signed-off-by: Steven Grimm <redacted>
Acked-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>