RFR: merge-bot: update merge conflict message

[Erik Helin]

Hi all,

please review this patch that updates the message in automatic merge conflict
pull requests. For an example, see <https://github.com/openjdk/panama-foreign/pull/49>.

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - merge-bot: update merge conflict message

Changes: https://git.openjdk.java.net/skara/pull/540/files
 Webrev: https://webrevs.openjdk.java.net/skara/540/webrev.00
  Stats: 52 lines in 1 file changed: 20 ins; 5 del; 27 mod
  Patch: https://git.openjdk.java.net/skara/pull/540.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/540/head:pull/540

PR: https://git.openjdk.java.net/skara/pull/540