Merge branch 'pc/lockfile-pid' into seen

Allow recording process ID of the process that holds the lock next
to a lockfile for diagnosis.

Comments?

* pc/lockfile-pid:
  lockfile: add PID file for debugging stale locks

diff --cc Documentation/config/core.adoc
Simple merge

diff --cc builtin/commit.c
Simple merge

diff --cc builtin/gc.c
Simple merge

diff --cc compat/mingw.c
Simple merge

diff --cc environment.c
Simple merge

diff --cc lockfile.c
Simple merge

diff --cc t/meson.build
Simple merge