Merge branch 'ma/more-opaque-lock-file' into maint

Code clean-up.

* ma/more-opaque-lock-file:
  read-cache: try not to peek into `struct {lock_,temp}file`
  refs/files-backend: don't peek into `struct lock_file`
  midx: don't peek into `struct lock_file`
  commit-graph: don't peek into `struct lock_file`
  builtin/gc: don't peek into `struct lock_file`