builtin/maintenance: introduce "reflog-expire" task

By default, git-maintenance(1) uses the "gc" task to ensure that the
repository is well-maintained. This can be changed, for example by
either explicitly configuring which tasks should be enabled or by using
the "incremental" maintenance strategy. If so, git-maintenance(1) does
not know to expire reflog entries, which is a subtask that git-gc(1)
knows to perform for the user. Consequently, the reflog will grow
indefinitely unless the user manually trims it.

Introduce a new "reflog-expire" task that plugs this gap:

  - When running the task directly, then we simply execute `git reflog
    expire --all`, which is the same as git-gc(1).

  - When running git-maintenance(1) with the `--auto` flag, then we only
    run the task in case the "HEAD" reflog has at least N reflog entries
    that would be discarded. By default, N is set to 100, but this can
    be configured via "maintenance.reflog-expire.auto". When a negative
    integer has been provided we always expire entries, zero causes us
    to never expire entries, and a positive value specifies how many
    entries need to exist before we consider pruning the entries.

Note that the condition for the `--auto` flags is merely a heuristic and
optimized for being fast. This is because `git maintenance run --auto`
will be executed quite regularly, so scanning through all reflogs would
likely be too expensive in many repositories.

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

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