Use contents of local checkout instead of last commit for validation

Validating only committed files is not useful in the natural
  $test_everything_passes; commit; push
workflow; the failures will not be caught locally, only by Travis later
(and only if PRs are used instead of direct commits to master).

So, use the working directory state instead of last commit for
validations; and remove misleading comments in checks which already use
the working directory state.

hack/make/validate-lint

@@ -11,7 +11,6 @@ unset IFS
 
 errors=()
 for f in "${files[@]}"; do
-	# we use "git show" here to validate that what's committed passes go lint
 	failedLint=$(golint "$f")
 	if [ "$failedLint" ]; then
 		errors+=( "$failedLint" )