IMPL: check that merge was not a fastforward or noop

Sometimes the new history happens to already contain the used-to-be-merged parent.
Then merge, repeated, will not do anything. Another, much more rare but still possible,
option is fastforward. Both this operation does not actually rewrite the history, and
probably would have been removed by user from history. If it's still here, it is probably
some mistake. Options which user would probably want to choose after stopping on this:
* force non-fastforward merge
* merge some other parent instead of the one from history
* skip the step at all (probably during non-interactive rebase)
DOC: improve report on the problematic merge