RFR: 2467: Skara bot mistakenly reset the committer when amending commit message

[Zhao Song]

Kevin Rushforth reported that this commit(https://github.com/openjdk/skara/commit/e0767b8b621b18c813ea579161791a836625d540) was sponsored by Magnus, so Magnus should be the committer, however, Tobias was set as the committer.

After investigation, I found that there is a stale review in the pr, so the bot amended the commit before pushing it. When amending the commit, the bot should only amend the commit message, however, it also reset the committer.

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Commit messages:
 - update
 - SKARA-2467

Changes: https://git.openjdk.org/skara/pull/1711/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1711&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2467
  Stats: 99 lines in 2 files changed: 92 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/skara/pull/1711.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1711/head:pull/1711

PR: https://git.openjdk.org/skara/pull/1711