Before submitting a pull 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