Add support for automerging without PR, Closes #177 

* update definitions and docs


* Add mergeBranch api

* support merge commit

* set automergeType

* Update API

* Refactor merge commit

* branch-push working

* Refactor branch

* Add back base tree

* Fix failing tests

* Update definitions and docs

* Fix automerge logic

* Test isBranchStale

* start mergeBranch test

* test mergeBranch branch-push throws

* more tests

* test unknown commit type

* pr tests

* Detect automerge in versions helper

* update tests for new automergeEnabled flag

* refactor pr logic

* complete pr worker tests

* branch automerge tests

* Update docs

* refactor branch automerge check

* Update dependency gh-got to version 6.0.0

* Fix lock file

* Update dependency np to version 2.16.0

* Fix yarn.lock

Accept `application/json` as content type fallback in case user is accessing
npm registry that does not support the new `npm.install` content type.

* Update base commit SHA after merge

* drop unnecessary getcommittree

This reverts commit 981d446c.

* Update dependency chai to version 4.0.2

* Update yarn

Closes #265

Partial implementation of #252 

* use snapshots

* Use npm cache

Closes #246

* Add test for caching

Closes #202 

* Externalise hbs templates

* Fix tests

* Move templates

* lint fix

* Pass all upgrades to ensurePr

* Group changelogs

* Fix lint

* Fix tests

* Refactor yarn/npm run

* Fix

* Externalise hbs templates

* Fix tests

* Move templates

* lint fix

* Refactor templates location

Closes #252, Closes #211

* Add robustness to package.json parsing

* Add tests

Closes #245

* Update test names

* Update yarn.lock