gc --auto: release pack files before auto packing

gc --auto: release pack files before auto packing

Teach gc --auto to release pack files before auto packing the repository
to prevent failures when removing them.

Also teach the test 'fetching with auto-gc does not lock up' to complain
when it is no longer triggering an auto packing of the repository.

Fixes https://github.com/git-for-windows/git/issues/500

Signed-off-by: Kim Gybels <kgybels@infogroep.be>
Signed-off-by: Junio C Hamano <gitster@pobox.com>