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