RFR: 1683: The force push notifier needs an overhaul [v2]

[Zhao Song]

> A user reported a bug related with the force-push notification. Erik investigated it and found the implementation of this notification should not be in the `PullRequestBranchNotifier`. Also, users preferred to not get force-push notification if the force-push happens when the pr is in draft state.
> 
> In this patch, the implementation of checking force-push and notifying user are moved to `CheckWorkItem`. Also, force-pushing or rebasing a draft pr will not warn user any more.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  fix some bugs

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1426/files
  - new: https://git.openjdk.org/skara/pull/1426/files/c5bbd151..aabd0b7a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1426&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1426&range=00-01

  Stats: 20 lines in 5 files changed: 1 ins; 1 del; 18 mod
  Patch: https://git.openjdk.org/skara/pull/1426.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1426/head:pull/1426

PR: https://git.openjdk.org/skara/pull/1426