1377 - Blacklist dependencies from SCM tracking

Git excludes dependencies from commit. 
Mercurial just shows a warning because it doesn't support excludes on commits. Just hgignore
Well, in that case it is probably there because either a) the user has created it manually or b) some weird issue happened during the invocation of `ensure_dependencies.py`. Either way, it requires manual intervention - and it might not be a good idea to have the script trying to resolve that.