RFR: 233: Merge bot should be able to multiple branches

[Erik Helin]

Hi all,

please review this patch that makes the merge bot being able to merge multiple
branches for a single target repository. For example, the merge bot can now be
set up to merge e.g. `jdk:master` into the target's `master` branch and then
merge the target's `master` branch into the target's `dev` branch. Merge
conflict pull request "alerts" are created per merge specification (from repo,
from branch, target branch).

Thanks,
Erik

## Testing
- Updated existing unit tests

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

Commits:
 - 63921e84: skara-233

Changes: https://git.openjdk.java.net/skara/pull/365/files
 Webrev: https://webrevs.openjdk.java.net/skara/365/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-233
  Stats: 220 lines in 3 files changed: 89 ins; 43 del; 88 mod
  Patch: https://git.openjdk.java.net/skara/pull/365.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/365/head:pull/365

PR: https://git.openjdk.java.net/skara/pull/365