RFR: merge-bot: no need to force push

[Erik Helin]

Hi all,

please review this small patch that makes the merge bot no longer use force
pushes. The redesigned merge bot has no need for force pushes, all "from"
branches (branches in bot's fork) will have a unique name, so there should never
be a conflict when pushing.

Testing:
- `make test` on Linux x64

Thanks,
Erik

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Commit messages:
 - merge-bot: no need to force push

Changes: https://git.openjdk.java.net/skara/pull/582/files
 Webrev: https://webrevs.openjdk.java.net/skara/582/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/582.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/582/head:pull/582

PR: https://git.openjdk.java.net/skara/pull/582