find_pack_entry(): document last_found_pack

find_pack_entry(): document last_found_pack

Add a comment at the declaration of last_found_pack and where it is
used in find_pack_entry().  In the latter, separate the cases (1) to
make a place for the new comment and (2) to turn the success case into
affirmative logic.

Signed-off-by: Michael Haggerty <redacted>
Reviewed-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>