builtin/maintenance: run maintenance tasks depending on type

builtin/maintenance: run maintenance tasks depending on type

We basically have three different ways to execute repository
maintenance:

  1. Manual maintenance via `git maintenance run`.

  2. Automatic maintenance via `git maintenance run --auto`.

  3. Scheduled maintenance via `git maintenance run --schedule=`.

At the moment, maintenance strategies only have an effect for the last
type of maintenance. This is about to change in subsequent commits, but
to do so we need to be able to skip some tasks depending on how exactly
maintenance was invoked.

Introduce a new maintenance type that discern between manual (1 & 2) and
scheduled (3) maintenance. Convert the `enabled` field into a bitset so
that it becomes possible to specifiy which tasks exactly should run in a
specific context.

The types picked for existing strategies match the status quo:

  - The default strategy is only ever executed as part of a manual
    maintenance run. It is not possible to use it for scheduled
    maintenance.

  - The incremental strategy is only ever executed as part of a
    scheduled maintenance run. It is not possible to use it for manual
    maintenance.

The strategies will be tweaked in subsequent commits to make use of this
new infrastructure.

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