Ensure beachball properly identifies deleted change-files from same author

[nickymunga]

The goal of this change is to ensure that beachball properly identifies a delete/add for change files done by the same author as Deleted instead of Rename. We're specifying a 100% threshold on the similarity index, meaning git will consider a delete/add pair a rename only if 0% of the file has changed. Beachball is currently using the default threshold of 50%, Meaning if someone deletes a change file, then creates a new one; git will consider this a rename. & `beachball check --disallow-deleted-change-files' will fail to capture this.

Scenario: Git History:

• running beachball change with similarity index: 50% :

• running beachball check with similarity index: 100%