Make it possible to run the same `multimod tag` command multiple times

[dmitryax]

It would be nice to be able to do a dry run of git push based on the output of multimod tag --print-tags, but it'll make the second run of multimod tag fail on the same commit because tags are already created. multimod tag potentially can be updated to check if the tags are already created on the specified commit just return a similar output instead of failing.

[aliakseip]

I am starting to work on this improvement.