Gc#deleteOrphans: avoid dependence on PackExt alphabetical ordering

Gc#deleteOrphans: avoid dependence on PackExt alphabetical ordering

Deleting orphan files depends on .pack and .keep being reverse-sorted
to before the corresponding index files that could be orphans. The new
reverse index file extension (.rev) will break that frail dependency.

Rewrite Gc#deleteOrphans to avoid that dependence by tracking which pack
names have a .pack or .keep file and then deleting any index files that
without a corresponding one. This approach takes linear time instead of
the O(n logn) time needed for sorting.

Change-Id: If83c378ea070b8871d4b01ae008e7bf8270de763
Signed-off-by: Anna Papitto <annapapitto@google.com>