The command line parameters for git diff between two branches look like:
git diff branchname..HEAD -- path/to/file
The parameters for this command are:
git diff-ansible-vault -r branchname -p path/to/file
It would be nicer if the parameters for this command were in line with the standard git form of parameters.
git diff-ansible-vault branchname..HEAD -- path/to/file
Other parameters such as -v, --no-color etc can remain unaltered, provided - as with other git commands - they happen before the -- divider.