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
Unfortunately it's a bit tricky. We need to manipulate (or create) the `[ui]` section in `.hg/hgrc`, introduce a new `ignore` directive pointing to a new `hgignore` file or similar. And that's not enough - if the user already has defined a `[ui]/ignore` directive in his global `$HOME/.hgrc`, we'd overwrite that one (just tested it).