RFR: 231: The merge bot cannot create merge conflict PRs

[Erik Helin]

Hi all,

please review this patch that fixes the merge bot. The merge bot is able to
automatically merge commits from a branch in one repository to a branch in
another repository. When the merge bot encounters a merge conflict it is
supposed to open a PR describing the conflict.

This didn't work because I forgot to include a personal fork for the bot when
writing the code the first time around. This patch fixes and makes the merge bot
aware of a personal fork it can use for creating PRs.

I also had to update `TestPullRequest` a little bit to make it work with pull
requests _between_ two different `TestHostedRepository` instances.

Finally I also added the new method `Repository.prune`. It isn't currently used
(besides by the unit test), but I used it at one point in time during this
rewrite of the merge bot and I have a feeling we might need it again soon, so I
decided to let it stay and be part of the patch.

Thanks,
Erik

## Testing
- [x] `make test` on Linux x64 (includes unit tests for the merge bot)

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

Commits:
 - 1bce3f13: skara-231

Changes: https://git.openjdk.java.net/skara/pull/361/files
 Webrev: https://webrevs.openjdk.java.net/skara/361/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-231
  Stats: 177 lines in 10 files changed: 122 ins; 1 del; 54 mod
  Patch: https://git.openjdk.java.net/skara/pull/361.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/361/head:pull/361

PR: https://git.openjdk.java.net/skara/pull/361