Automerge bugs

* use git fetch + reset hard instead of git pull
* remove usage of 'git pull --rebase'. A rebase is never the right thing to do after committing a merge.
* refactored the GitMergeExecutor to be easily retryable
* Added deletion of branches after they have been auto merged by Kochiku

@square/kochiku-contributors 
Deleting the branch automatically works for me but I wonder whether there will be users who don't expect it and find it surprising. Maybe a mouseover on the check box which explains that is what this does?