Script for merging changes from a bugfix branch into master (#11299)

* Supports "Merge: no" for changesets which should be skipped, e.g. release notes updates.
* Supports "Merge: manual" which will causes the merge to stop, requiring somebody to manually merge to change before continuing

Change-Id: If5d1291ec77f4c1e31b3afb8420a45ecceedfa15