Merge branch 'ps/maintenance-daemonize-lockfix'

"git maintenance" that goes background did not use the lockfile to
prevent multiple maintenance processes from running at the same
time, which has been corrected.

* ps/maintenance-daemonize-lockfix:
  run-command: honor "gc.auto" for auto-maintenance
  builtin/maintenance: fix locking with "--detach"

diff --cc setup.c
Simple merge

diff --cc t/t7900-maintenance.sh
Simple merge