Merge branch 'ma/roll-back-lockfiles'

Some codepaths used to take a lockfile and did not roll it back;
they are automatically rolled back at program exit, so there is no
real "breakage", but it still is a good practice to roll back when
you are done with a lockfile.

* ma/roll-back-lockfiles:
  sequencer: do not roll back lockfile unnecessarily
  merge: always roll back lock in `checkout_fast_forward()`
  merge-recursive: always roll back lock in `merge_recursive_generic()`
  sequencer: always roll back lock in `do_recursive_merge()`
  sequencer: make lockfiles non-static
