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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state