builtin/maintenance: make "gc" strategy accessible

builtin/maintenance: make "gc" strategy accessible

While the user can pick the "incremental" maintenance strategy, it is
not possible to explicitly use the "gc" strategy. This has two
downsides:

  - It is impossible to use the default "gc" strategy for a specific
    repository when the strategy was globally set to a different strategy.

  - It is not possible to use git-gc(1) for scheduled maintenance.

Address these issues by making making the "gc" strategy configurable.
Furthermore, extend the strategy so that git-gc(1) runs for both manual
and scheduled maintenance.

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