RFR: Add merge bot

[Erik Duveblad via github.com]

Hi,

this patch adds a merge bot. The merge automatically tries to merge a branch from one repository to another. If the merge fails then the bot will open a PR describing the commits that couldn't be merge. Once the conflicts have been resolved and pushed/integrated by a human contributor, then the bot will automatically close the PR.

## Testing
- [x] `sh gradlew test` passes on Linux x86_64
- [x] Added four new unit tests

Thanks,
Erik

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Commits:
 - 7fc5a814:	Add merge bot

Pull request:
https://git.openjdk.java.net/skara/pull/104

Webrev:
https://webrevs.openjdk.java.net/skara/104/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/104.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/104/head:pull/104