- replace notification.type.pull.request to notification.type.pullrequest

This fixes the evolution error which occurs when start the application
in production mode.

The value was set to false at f08b546 to prevent losing of your
database, but it is not needed anymore because currently database
is stored as a file not into memory.

The fix of be9a838 was incorrect. PullRequest's diff should compare
merged commit and its parent, not its left parent and right parent.

Remove unnecessary @defining clause which causes the exception because
of calling PullRequest.getDiff.

Commit ids for the diff was incorect.