[Refactoring] Split processing in onChangeEvent
[Refactoring] Rename processAtomicCommit

Check that the commit is atomic before calling the method.
Remove wrong log.
[Refactoring] replace 'why' String with a boolean

Content of 'why' was not used.
[Refactoring] extract merge of a review
[Refactoring] mutualize check of review existence
[Refactoring] extract attemptToMerge method
Avoid unclear exception when no bot user exists.
[Clean-up] Remove unused variables and imports
[Bug fix] Fix infinite loop

When the bot puts a comment, it is processed by the plugin,
leading to an infinite loop.
[Cosmetic] Add comment
[Refactoring] factorize getRevisionResource
[Bug fix] Review all comments that don't come from the plugin

If the bot puts "Verified +1", we must not return as the review can become mergeable.
[Bug fix] bot must not Code-review -2

We replace Code-review -2 by -1 as we don't want the -2 to stay forever.
The -1 will just be a warning that will disappear when there is a new patch set.
Anyway, the presubmit will fail thanks to devtools-cli. So we can't have Verified +1.
[Bug fix] fix check on comment prefix

The patchet set number is written before the prefix.
Merge pull request #1 from criteo/fix-infinite-loop

Fix infinite loop
[Refactor] Makes the merge strategy a bit clearer.

And fixes a bug when a review was mergeable but another one was not
submittable yet.
Add scenarios for manual tests.
Fix wrong comment on simple reviews

When a review is not cross-repo and cannot be merged because of a conflict,
we were showing a message prefixed with "Cross-repo comment".
Now, the messages are no more prefixed.
Fix NPE when processing comment