RFR: 1658: Minimize race window when updating PR body

[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.

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Commit messages:
 - SKARA-1658

Changes: https://git.openjdk.org/skara/pull/1416/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1416&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1658
  Stats: 56 lines in 8 files changed: 53 ins; 0 del; 3 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