gc: Add options to preserve and prune old pack files

gc: Add options to preserve and prune old pack files

The new --preserve-oldpacks option moves old pack files into the
preserved subdirectory instead of deleting them after repacking.

The new --prune-preserved option prunes old pack files from the
preserved subdirectory after repacking, but before potentially
moving the latest old packfiles to this subdirectory.

These options are designed to prevent stale file handle exceptions
during git operations which can happen on users of NFS repos when
repacking is done on them. The strategy is to preserve old pack files
around until the next repack with the hopes that they will become
unreferenced by then and not cause any exceptions to running processes
when they are finally deleted (pruned).

Change-Id: If3f729f0d9ce920ee2c3e6acdde46f2068be61d2
Signed-off-by: James Melvin <jmelvin@codeaurora.org>