Configure Phabricator to use a staging area.

[ChristianKuehnel]

basic idea

To properly be able to test the patches people upload, set up a "staging area" in Phabricator for the github monorepo. This way contributors upload the exact code they have locally, so that the build can be reproduced by the pre-merge tests:
https://secure.phabricator.com/book/phabricator/article/harbormaster/

This is based on a bug reported in #64 .

Drawbacks

• This might have impact on contributor's workflows.
• We need to set up and maintain the stating repository.
• This still does not guarantee, that you patch can be cleanly applied to the master branch at the time of merging.
• People still can merge somthing different from what was reviewed and tested.

alternative solution

Use github pull requests. This would solve all of the problems above except: It has an even bigger impact on the workflows.

[ChristianKuehnel]

won't fix because moving towards #154