obsolete: always return frozensets from obsolete.getrevs() getrevs function already returns an empty frozenset when there is no obsstore, but let's make sure to return a frozenset in any case. This makes it possible to use the result of this function as a dict key or provide it to hash() built-in function without any conversions. Differential Revision: https://phab.mercurial-scm.org/D12156