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
No, not at all. I meant iterating and applying that one to all `repo_types` because one may *use them in parallel*. So e.g. `for scm in repo_types.itervalues(): if scm.istype(repodir): scm.ignore(target)`..