Merge branch 'ps/maintenance-geometric'

Merge branch 'ps/maintenance-geometric'

"git maintenance" command learns the "geometric" strategy where it
avoids doing maintenance tasks that rebuilds everything from
scratch.

* ps/maintenance-geometric:
  t7900: fix a flaky test due to git-repack always regenerating MIDX
  builtin/maintenance: introduce "geometric" strategy
  builtin/maintenance: make "gc" strategy accessible
  builtin/maintenance: extend "maintenance.strategy" to manual maintenance
  builtin/maintenance: run maintenance tasks depending on type
  builtin/maintenance: improve readability of strategies
  builtin/maintenance: don't silently ignore invalid strategy
  builtin/maintenance: make the geometric factor configurable
  builtin/maintenance: introduce "geometric-repack" task
  builtin/gc: make `too_many_loose_objects()` reusable without GC config
  builtin/gc: remove global `repack` variable