Merge branch 'jk/gc-pre-detach-under-hook' into maint

We run an early part of "git gc" that deals with refs before daemonising (and not under lock) even when running a background auto-gc, which caused multiple gc processes attempting to run the early part at the same time. This is now prevented by running the early part also under the GC lock. * jk/gc-pre-detach-under-hook: gc: run pre-detach operations under lock