CosmeticChangesBot shows diff twice

Rename CosmeticChangesToolkit option 'debug' to 'diff', as that
is its only function.

CosmeticChangesBot should call CosmeticChangesToolkit with the diff
option disabled, as userPut will show the diff.

Change-Id: Ia44ca916e5aaaef08f9b4ea5ebcdcd24cddf3f08
Merge "CosmeticChangesBot shows diff twice"