rollback: show that the safety works in a associated test

That test section is checking that --force is overcoming the safety check,
however we did not check that the safety properly detect the situation.

We now do for clarity.

tests/test-rollback.t

@@ -76,6 +76,10 b' working dir unaffected by rollback: do n'
   (activating bookmark bar)
   $ cat .hg/undo.branch ; echo
   test
+  $ hg rollback
+  abort: rollback of last commit while not checked out may lose data
+  (use -f to force)
+  [255]
   $ hg rollback -f
   repository tip rolled back to revision 1 (undo commit)
   $ hg id -n