RFR: 1239: The backport command bot replies multiple `@username` unexpectedly

[Guoxiong Li]

Hi all,

After [SKARA-1201](https://bugs.openjdk.java.net/browse/SKARA-1201), the backport command bot will reply multiple `@username` unexpectedly when we use command `/backport branch-name`.

Please see [this link](https://github.com/openjdk/jdk/commit/2b02b6f513b062261195ca1edd059d16abb7bec6) for the detailed messages. The bot replied two `@lgxbslgx` unexpectedly.

This patch fixes the issue and adds the corresponding test cases.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - SKARA-1239: The backport command bot replies multiple @username unexpectedly

Changes: https://git.openjdk.java.net/skara/pull/1238/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1238&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1239
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1238.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1238/head:pull/1238

PR: https://git.openjdk.java.net/skara/pull/1238