rebase: explicitly test abort from ambiguous destination

We want to explicitly test the next behavior. We add this test in its own
changeset to make the test change from the behavior change as clean as possible.

tests/test-rebase-parameters.t

@@ -82,6 +82,11 b' These fail:'
   abort: cannot specify both a revision and a base
   [255]
 
+  $ hg rebase --base 6
+  abort: branch 'default' has 3 heads - please rebase to an explicit rev
+  (run 'hg heads .' to see heads)
+  [255]
+
   $ hg rebase --rev '1 & !1' --dest 8
   empty "rev" revision set - nothing to rebase
   [1]