git-gc --auto: protect ourselves from accumulated cruft

git-gc --auto: protect ourselves from accumulated cruft

Deciding to run "repack -d -l" when there are too many
loose objects would backfire when there are too many loose
objects that are unreachable, because repacking that way would
never improve the situation.  Detect that case by checking the
number of loose objects again after automatic garbage collection
runs, and issue an warning to run "prune" manually.

Signed-off-by: Junio C Hamano <gitster@pobox.com>