Add conflict resolution options to git-cms-merge-topic.

[ktf]

From @vadler

git-cms-merge-topic is a very useful tool, but it lacks the
functionality to continue after solvinf conflicts so far. Usually, I
solve the conflicts, commit the solutions and execute the lines after
'git merge --no-ff [...]' manually.
It would be great, if a corresponding option "-c/--continue" (and also
"-a/--abort") could be added.