gc: add `gc.repackFilterTo` config option

gc: add `gc.repackFilterTo` config option

A previous commit implemented the `gc.repackFilter` config option
to specify a filter that should be used by `git gc` when
performing repacks.

Another previous commit has implemented
`git repack --filter-to=<dir>` to specify the location of the
packfile containing filtered out objects when using a filter.

Let's implement the `gc.repackFilterTo` config option to specify
that location in the config when `gc.repackFilter` is used.

Now when `git gc` will perform a repack with a <dir> configured
through this option and not empty, the repack process will be
passed a corresponding `--filter-to=<dir>` argument.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>