RFR: 1069: PR ends up in bad state if interrupted just after push

[Erik Joelsson]

This change adds a new transaction step when integrating a PR, to better handle if the bot gets interrupted mid integration. Currently, if the change is pushed and the bot is interrupted before closing, changing labels or adding the final "commit pushed" comment, the PR can end up in a limbo state.

The new step is another comment "Going to push commit as ..." which gets added right before the git push command is run. Using this comment, it's now possible to automatically recover if the bot gets interrupted. The integration command checks for any such comments and if found, checks if the commit hash is present in the target. If it is, the PR was already pushed, and the command will just close it out as normal.

I also decided to move the output of any rebase command to this prepush comment, so we don't risk losing it.

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Commit messages:
 - SKARA-1069

Changes: https://git.openjdk.java.net/skara/pull/1183/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1183&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1069
  Stats: 261 lines in 5 files changed: 242 ins; 14 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/1183.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1183/head:pull/1183

PR: https://git.openjdk.java.net/skara/pull/1183