branch: delete obsolete message about changing branch of obsolete commit

We now rely on `rewriteutil.precheck()` to check for divergence, so we
don't need the extra check in `cmdutil.changebranch()`. The former
check is a little less strict in that it allows you to rewrite a
commit without non-obsolete successors.

Differential Revision: https://phab.mercurial-scm.org/D10519

mercurial/cmdutil.py

@@ -997,11 +997,6 b' def changebranch(ui, repo, revs, label, '
                 _(b"a branch of the same name already exists")
             )
 
-        if repo.revs(b'obsolete() and %ld', revs):
-            raise error.InputError(
-                _(b"cannot change branch of a obsolete changeset")
-            )
-
         # make sure only topological heads
         if repo.revs(b'heads(%ld) - head()', revs):
             raise error.InputError(