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: NMichael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: NNicolas Pitre <nico@fluxnic.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>