unstable: use the `_mutablerevs` function when computing phase divergent

This is a useful function to get the revision relevant to these computation,
lets make sure all code use it, so that we can improve that `_mutablerevs`
function in a later changeset.

mercurial/obsolete.py

@@ -993,7 +993,8 b' def _computephasedivergentset(repo):'
     torev = cl.index.get_rev
     tonode = cl.node
     obsstore = repo.obsstore
-    for rev in repo.revs(b'(not public()) and (not obsolete())'):
+    candidates = sorted(_mutablerevs(repo) - getrevs(repo, b"obsolete"))
+    for rev in candidates:
         # We only evaluate mutable, non-obsolete revision
         node = tonode(rev)
         # (future) A cache of predecessors may worth if split is very common