[chore] Set some additional git attributes (#3454)

* .go files are now tagged with diff=go for better diffs
* go.sum is tagged with generated which suppresses the diff
* go.sum is tagged with merge=ours to reduce merge conflicts

go.mod isn't tagged with generated since it's useful to see it on
dependency bump PRs.

.gitattributes

@@ -1 +1,3 @@
-/vendor/ linguist-generated
+*.go                diff=golang
+go.sum              linguist-generated merge=ours
+/vendor/            linguist-generated