hack: add apidiff.sh

apidiff can be invoked for a single internal package or iterate over
everything, including staging. The base to compare against can be specified via
-r (similar to verify-golangci-lint.sh), with the default the base revision on
master (similar to -a in verify-golangci-lint.sh).

hack/tools/tools.go

@@ -36,4 +36,5 @@ import (
 
 	// tools like cpu
 	_ "go.uber.org/automaxprocs"
+	_ "golang.org/x/exp/cmd/apidiff"
 )