obsolete: ensure that `getrevs` always return a set

When a set of revisions was empty, we were using an empty tuple. We now return an
empty frozenset to ensure the object could be used in an operation that requires a
set.

mercurial/obsolete.py

@@ -852,7 +852,7 b' def getrevs(repo, name):'
     Such access may compute the set and cache it for future use"""
     repo = repo.unfiltered()
     if not repo.obsstore:
-        return ()
+        return frozenset()
     if name not in repo.obsstore.caches:
         repo.obsstore.caches[name] = cachefuncs[name](repo)
     return repo.obsstore.caches[name]