Before submitting a pul request, I'd encourage you to test it yourself.
To do so, you need to run the plugin indirectly or directly-
Indirect Install the plugin locally and run it via helm:
$ helm plugin uninstall diff
$ helm plugin list
#=> Make sure that the previous installation of helm-diff has unisntalled
$ make install
$ helm plugin list
#=> Make sure that the version of helm-diff built from your branch has instaled
$ helm diff upgrade ... (snip)
Direct Build the plugin binary and execute it with a few helm-specific environment variables:
$ go build .
$ HELM_NAMESPACE=default \
HELM_BIN=helm372 \
./helm-diff upgrade foo $CHART \
--set argo-cd.nameOverride=testtest \
--install