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.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a