wire: add a diff command

[vangent]

Fixes #725.

Sample output:

$ wire diff ./...
github.com/google/go-cloud/samples/guestbook: diff from /usr/local/google/home/rvangent/go/src/github.com/google/go-cloud/samples/guestbook/wire_gen.go:
@@ -14,0 +15 @@
+	"github.com/google/go-cloud/aws/rds"
@@ -40 +41 @@
-	certFetcher := &rdsmysql.CertFetcher{
+	certFetcher := &rds.CertFetcher{
@@ -213,0 +215,2 @@
+// awsBucket is a Wire provider function that returns the S3 bucket based on the
+// command-line flags.
@@ -217,0 +221,3 @@
+// awsSQLParams is a Wire provider function that returns the RDS SQL connection
+// parameters based on the command-line flags. Other providers inside
+// awscloud.AWS use the parameters to construct a *sql.DB.
@@ -226,0 +233,2 @@
+// awsMOTDVar is a Wire provider function that returns the Message of the Day
+// variable from SSM Parameter Store.
@@ -234,0 +243,2 @@
+// gcpBucket is a Wire provider function that returns the GCS bucket based on
+// the command-line flags.
@@ -238,0 +249,3 @@
+// gcpSQLParams is a Wire provider function that returns the Cloud SQL
+// connection parameters based on the command-line flags. Other providers inside
+// gcpcloud.GCP use the parameters to construct a *sql.DB.
@@ -249,0 +263,2 @@
+// gcpMOTDVar is a Wire provider function that returns the Message of the Day
+// variable from Runtime Configurator.
@@ -266,0 +282,2 @@
+// localBucket is a Wire provider function that returns a directory-based bucket
+// based on the command-line flags.
@@ -270,0 +288,2 @@
+// dialLocalSQL is a Wire provider function that connects to a MySQL database
+// (usually on localhost).
@@ -282,0 +302,2 @@
+// localRuntimeVar is a Wire provider function that returns the Message of the
+// Day variable based on a local file.