RFR: merge-bot: recreate repository on failed push

[Erik Helin]

Hi all,

please review this patch that makes the merge bot recreate the repository on a
failed push. A push can fail for a variety of reasons: network errors,
conflicting push by another contributor, etc. The safest, and easiest, way to
handle this scenario is to just recreate the repository. To make this operation
cheap I made the merge bot utilize a `HostedRepositoryPool`.

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - merge-bot: recreate repository on failed push

Changes: https://git.openjdk.java.net/skara/pull/570/files
 Webrev: https://webrevs.openjdk.java.net/skara/570/webrev.00
  Stats: 57 lines in 1 file changed: 35 ins; 20 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/570.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/570/head:pull/570

PR: https://git.openjdk.java.net/skara/pull/570