RFR: merge-bot: create integrateable pull requests

[Erik Helin]

Hi all,

please review this patch that makes the merge bot creates pull requests that can be integrated. With this new version
of the merge bot contributors will be able to update the "merge style" PR created by the automatic merge bot. This
should reduce the overhead quite a bit for resolving a merge conflict.

Testing:
- `make test`
- Updated unit tests

Thanks,
Erik

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

Commit messages:
 - Use (most likely) PR id as branch name
 - merge-bot: create real merge PRs and integrate them

Changes: https://git.openjdk.java.net/skara/pull/557/files
 Webrev: https://webrevs.openjdk.java.net/skara/557/webrev.00
  Stats: 261 lines in 13 files changed: 69 ins; 125 del; 67 mod
  Patch: https://git.openjdk.java.net/skara/pull/557.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/557/head:pull/557

PR: https://git.openjdk.java.net/skara/pull/557