Merge branch 'ps/maintenance-ref-lock'

"git maintenance" lacked the care "git gc" had to avoid holding onto the repository lock for too long during packing refs, which has been remedied. * ps/maintenance-ref-lock: builtin/maintenance: fix locking race when handling "gc" task builtin/gc: avoid global state in `gc_before_repack()` usage: allow dying without writing an error message builtin/maintenance: fix locking race with refs and reflogs tasks builtin/maintenance: split into foreground and background tasks builtin/maintenance: fix typedef for function pointers builtin/maintenance: extract function to run tasks builtin/maintenance: stop modifying global array of tasks builtin/maintenance: mark "--task=" and "--schedule=" as incompatible builtin/maintenance: centralize configuration of explicit tasks builtin/gc: drop redundant local variable builtin/gc: use designated field initializers for maintenance tasks