builtin/gc: add a `--detach` flag

builtin/gc: add a `--detach` flag

When running `git gc --auto`, the command will by default detach and
continue running in the background. This behaviour can be tweaked via
the `gc.autoDetach` config, but not via a command line switch. We need
that in a subsequent commit though, where git-maintenance(1) will want
to ask its git-gc(1) child process to not detach anymore.

Add a `--[no-]detach` flag that does this for us.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>