Steps toward merge resumption

* Added three mods of operation: start (default), resume, abort
* Blobs for test cases are now based on master so that files that aren't
  changed don't get listed as removed.
* New branch names for merge branches. Base commit, specified by user on
  command line and assumed to be 'master' by default, also has a new
  branch pointing at it for merge resumption support.
* Refactoring to support merge resumption.
* How merge conflicts are written to disk is improved:
  * MERGE_HEAD and MERGE_MSG written to special files in .git