feat: Add the diff command

[kyleconroy]

Add a diff command to easily check if sqlc generate has been run. The idea is to run this as part of your continuous integration workflow to verify that your generated code is always up to date. This won't work if you manually edit the contents of generated files.

Sample output

--- a/go/models.go
+++ b/go/models.go
@@ -13,3 +13,8 @@
 	Name string
 	Bio  sql.NullString
 }
+
+type Book struct {
+	ID    int64
+	Title string
+}
--- a/go/query.sql.go
+++ b/go/query.sql.go
@@ -31,16 +31,6 @@
 	return i, err
 }
 
-const deleteAuthor = `-- name: DeleteAuthor :exec
-DELETE FROM authors
-WHERE id = $1
-`
-
-func (q *Queries) DeleteAuthor(ctx context.Context, id int64) error {
-	_, err := q.db.ExecContext(ctx, deleteAuthor, id)
-	return err
-}
-
 const getAuthor = `-- name: GetAuthor :one
 SELECT id, name, bio FROM authors
 WHERE id = $1 LIMIT 1
@@ -55,7 +45,7 @@
 
 const listAuthors = `-- name: ListAuthors :many
 SELECT id, name, bio FROM authors
+ORDER BY bio
-ORDER BY name
 `
 
 func (q *Queries) ListAuthors(ctx context.Context) ([]Author, error) {
@@ -80,3 +70,14 @@
 	}
 	return items, nil
 }
+
+const selectOne = `-- name: SelectOne :one
+SELECT 1
+`
+
+func (q *Queries) SelectOne(ctx context.Context) (interface{}, error) {
+	row := q.db.QueryRowContext(ctx, selectOne)
+	var column_1 interface{}
+	err := row.Scan(&column_1)
+	return column_1, err
+}