RFR: 1658: Minimize race window when updating PR body [v3]

[Zhao Song]

> Currently, when the pr bot tries to update the PR body, users are also likely to update the PR body at the same time. Therefore, users' updates are likely to be overwritten by bot.  
> 
> This is a race between the bot and user and the race window is a little big right now.
> 
> To reduce the race window, in this patch, when PR bot is trying to update PR body, it will check whether the PR body has been modified by user. And if the PR bot has been modified, PR bot would not overwrite it.

Zhao Song has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into SKARA-1658
 - Added an automatic test for latestBody
 - SKARA-1658

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1416/files
  - new: https://git.openjdk.org/skara/pull/1416/files/d2194a68..8536fe98

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1416&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1416&range=01-02

  Stats: 796 lines in 34 files changed: 385 ins; 142 del; 269 mod
  Patch: https://git.openjdk.org/skara/pull/1416.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1416/head:pull/1416

PR: https://git.openjdk.org/skara/pull/1416