odb: run "pre-auto-gc" hook for all maintenance tasks

odb: run "pre-auto-gc" hook for all maintenance tasks

The "pre-auto-gc" hook is supposed to run before auto-maintenance
starts. The intent of this is to give users the ability to intercept
running maintenance in case there's for example an event that is not
supposed to run in parallel with repository maintenance.

This hook runs via `need_to_gc()`, which is invoked via two paths:

  - It is called directly by git-gc(1).

  - It is called indirectly by git-maintenance(1) via the "gc" task.

While the former makes sense, the latter is somewhat off. While the hook
is indeed strongly tied to gc'ing a repository, the original intent of
the hook is rather to inhibit any kind of automated garbage collection.
That noticeably also includes all the other maintenance tasks that our
new infrastructure may run, but those aren't getting intercepted at all.
The move towards our new maintenance strategy has thus somewhat neutered
the effectiveness of the hook.

Fix this issue by running the hook before the first auto-maintenance
task that would run as determined by the tasks's auto condition. Note
that this requires us to lift the call to `run_hooks()` out of
`needs_to_gc()`, as the hook would otherwise potentially run multiple
times.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>