Merge branch 'jc/reopen-lock-file'

There are cases where you lock and open to write a file, close it to
show the updated contents to external processes, and then have to
update the file again while still holding the lock, but the lockfile
API lacked support for such an access pattern.

* jc/reopen-lock-file:
  lockfile: allow reopening a closed but still locked file

diff --cc cache.h
Simple merge

diff --cc lockfile.c
Simple merge