RFR: 657: Integrate command uses "merge request" in failure message

[Erik Helin]

Hi all,

please review this patch that uses the wording "integration request" instead of "merge request" in failure messages for
the `/integrate` pull request command.

Testing:
- [x] `make test` passes on Linux x64
- [x] Updated unit tests

Thanks,
Erik

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Commit messages:
 - 657

Changes: https://git.openjdk.java.net/skara/pull/835/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=835&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-657
  Stats: 8 lines in 4 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/skara/pull/835.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/835/head:pull/835

PR: https://git.openjdk.java.net/skara/pull/835