revset: fix the definition of "unstable changesets" for "unstable" predicate

unstable-ness of changesets should be determined by obsolete-ness of
not descendants but ancestors.

mercurial/revset.py

@@ -1424,7 +1424,7 b' def tagged(repo, subset, x):'
 
 def unstable(repo, subset, x):
     """``unstable()``
-    Unstable changesets are non-obsolete with obsolete descendants."""
+    Unstable changesets are non-obsolete with obsolete ancestors."""
     # i18n: "unstable" is a keyword
     getargs(x, 0, 0, _("unstable takes no arguments"))
     unstableset = set(repo.revs('(obsolete()::) - obsolete()'))