backout: improve confusing 'cannot backout change on a different branch' abort

These days 'branch' usually refer to a named branch.

Instead, abort with 'cannot backout change that not is an ancestor'.

mercurial/commands.py

@@ -439,7 +439,7 b' def backout(ui, repo, node=None, rev=Non'
     op1, op2 = repo.dirstate.parents()
     a = repo.changelog.ancestor(op1, node)
     if a != node:
-        raise util.Abort(_('cannot backout change on a different branch'))
+        raise util.Abort(_('cannot backout change that is not an ancestor'))
 
     p1, p2 = repo.changelog.parents(node)
     if p1 == nullid:

tests/test-backout.t

@@ -92,7 +92,7 b' across branch'
 should fail
 
   $ hg backout 1
-  abort: cannot backout change on a different branch
+  abort: cannot backout change that is not an ancestor
   [255]
   $ echo c > c
   $ hg ci -Am2
@@ -108,7 +108,7 b' should fail'
 should fail
 
   $ hg backout 1
-  abort: cannot backout change on a different branch
+  abort: cannot backout change that is not an ancestor
   [255]
   $ hg summary
   parent: 2:db815d6d32e6 tip