RefService.DeleteRefs: implement `except_with_prefix`
This is a follow-up to #49 (closed) (see there for details): except_with_prefix
is used by only one call site on the Rails side, and we explicitly skip it for Mercurial projects.
This is a follow-up to #49 (closed) (see there for details): except_with_prefix
is used by only one call site on the Rails side, and we explicitly skip it for Mercurial projects.