builtin/maintenance: split into foreground and background tasks

Both git-gc(1) and git-maintenance(1) have logic to daemonize so that
the maintenance tasks are performed in the background. git-gc(1) has
some special logic though to not perform _all_ housekeeping tasks in the
background: both references and reflogs are still handled synchronously
in the foreground.

This split exists because otherwise it may easily happen that git-gc(1)
keeps the "packed-refs" file locked for an extended amount of time,
where the next Git command that wants to modify any reference could now
fail. This was especially important in the past, where git-gc(1) was
still executed directly as part of our automatic maintenance: git-gc(1)
was invoked via `git gc --auto --detach`, so we knew to handle most of
the maintenance tasks in the background while doing those parts that may
cause locking issues in the foreground.

We have since moved to git-maintenance(1), which is a more flexible
replacement for git-gc(1). By default this command runs git-gc(1), only,
but it can be configured to run different tasks, as well. This command
does not know about the split between maintenance tasks that should run
before and after detach though, and this has led to several bug reports
about spurious locking errors for the "packed-refs" file.

Prepare for a fix by introducing this split for maintenance tasks. Note
that this commit does not yet change any of the tasks, so there should
not (yet) be a change in behaviour.

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

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