Merge branch 'ps/maintenance-ref-lock' into jch

"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

diff --cc builtin/fetch.c
Simple merge

diff --cc builtin/gc.c
Simple merge

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc usage.c
Simple merge