* Improve BatchRefUpdateTest readability
* Move BatchRefUpdate tests to a new file
* Implement atomic BatchRefUpdates for RefDirectory
* Separate RefUpdate.Result.REJECTED_{MISSING_OBJECT,OTHER_REASON}
* TestRepository: Allow setting explicit Change-Id
* TestRepository: Ignore existing Change-Id in message
* TestRepository: Support committing to unborn symref
* TestRepository: Expose some getters for CommitBuilder
* TestRepository: Add methods to amend commits or refs
* TestRepository: Add a reset method to move HEAD around
* TestRepository: Optionally insert Change-Id in commit message