Merge branch 'sb/ref-lock-lose-lock-fd'

Merge branch 'sb/ref-lock-lose-lock-fd'

The refs API uses ref_lock struct which had its own "int fd", even
though the same file descriptor was in the lock struct it contains.
Clean-up the code to lose this redundant field.

* sb/ref-lock-lose-lock-fd:
  refs.c: remove lock_fd from struct ref_lock