Merge branch 'ps/gc-stale-lock-warning'

Give a bit of advice/hint message when "git maintenance" stops finding a
lock file left by another instance that still is potentially running.

* ps/gc-stale-lock-warning:
  t7900: fix host-dependent behaviour when testing git-maintenance(1)
  builtin/gc: provide hint when maintenance hits a stale schedule lock

diff --cc builtin/gc.c
Simple merge

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