RFR: Add topological merge bot

[Jorn Vernee via github.com]

The pull request has been updated with additional changes.

----------------

Added commits:
 - 7df299ef:	Addressing review comment
 - fc6b5f53:	Moved topological sort to it's own file, and added tests

Pull request:
https://git.openjdk.java.net/skara/pull/105

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/105/webrev.01
 - inc: https://webrevs.openjdk.java.net/skara/105/webrev.00-01

Updated full patch:
https://git.openjdk.java.net/skara/pull/105.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/105/head:pull/105