RFR: 1828: RFR emails still be sent when PR is in draft state

[Guoxiong Li]

Hi all,

This patch blocks the RFR email when new commits are pushed to a draft PR. And the test case is added. too.

But it doesn't solve the synchronization problem that Erik mentioned in [PR-1469](https://github.com/openjdk/skara/pull/1469#issuecomment-1425916242). Because the bot only generates one email one time regradless of the number of the commits.

Thanks for the review.

Best Regards,
-- Guoxiong

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Commit messages:
 - SKARA-1828

Changes: https://git.openjdk.org/skara/pull/1484/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1484&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1828
  Stats: 31 lines in 2 files changed: 30 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1484.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1484/head:pull/1484

PR: https://git.openjdk.org/skara/pull/1484