revset: use appropriate predicate name in error messages

"extinct" and "unstable" predicates use "obsolete" implementation
internally, but own predicate name should be used in error messages of
them instead of "obsolete".

mercurial/revset.py

@@ -615,7 +615,7 b' def draft(repo, subset, x):'
 def extinct(repo, subset, x):
     """``extinct()``
     obsolete changeset with obsolete descendant only."""
-    getargs(x, 0, 0, _("obsolete takes no arguments"))
+    getargs(x, 0, 0, _("extinct takes no arguments"))
     extinctset = set(repo.revs('(obsolete()::) - (::(not obsolete()))'))
     return [r for r in subset if r in extinctset]
 
@@ -1406,7 +1406,7 b' def tagged(repo, subset, x):'
 def unstable(repo, subset, x):
     """``unstable()``
     Unstable changesets are non-obsolete with obsolete descendants."""
-    getargs(x, 0, 0, _("obsolete takes no arguments"))
+    getargs(x, 0, 0, _("unstable takes no arguments"))
     unstableset = set(repo.revs('(obsolete()::) - obsolete()'))
     return [r for r in subset if r in unstableset]