[merge tool]: ensure latest version of ng-dev is locally used by caretakers

[devversion]

Similar to what we enforce for the publish tool we should have a check for the merge tool because:

• The caretaker might run on an outdated local version
• The local version could not follow the latest branching/versioning changes, which results in PRs potentially being merged into wrong branches. etc.

For the merge tool there are special requirements:

• We should not enforce that the tool is run from the latest revision upstream. This would mean the caretaker needs to rebase/pull whenever he merged something.

To achieve this, we thought about just fetching the lock file upstream and comparing the local ng-dev version that way. Draft: https://gist.github.com/devversion/f34d748ebd7ae02602c6a35d2e2c3013

This is unfortunately not 100% accurate though as it doesn't capture when the .ng-dev configuration has changed! so we need to come up with something better most likely. cc. @josephperrott