RFR: merge-bot: use commitMetadata when listing commits

[Erik Helin]

Hi all,

please review this patch that makes the automatic merge bot use `commitMetadata`
instead of `commits` to speed up listing of large commit ranges.

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - merge-bot: use commitMetadata when listing commits

Changes: https://git.openjdk.java.net/skara/pull/578/files
 Webrev: https://webrevs.openjdk.java.net/skara/578/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/578.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/578/head:pull/578

PR: https://git.openjdk.java.net/skara/pull/578