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
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..