Add --debug-check cli option (#627)

This let's you run `pp(x) === pp(pp(x))` across an entire codebase quickly. I think it's small enough that we can merge it in to master while we are nailing down all the issues.

You can see the output in #610

bin/prettier.js

@@ -23,6 +23,7 @@ const argv = minimist(process.argv.slice(2), {
     "help",
     "version",
     "debug-print-doc",
+    "debug-check",
     // Deprecated in 0.0.10
     "flow-parser"
   ],
@@ -131,6 +132,17 @@ function format(input) {
     const doc = prettier.__debug.printToDoc(input, options);
     return prettier.__debug.formatDoc(doc);
   }
+
+  if (argv["debug-check"]) {
+    const pp = prettier.format(input, options);
+    const pppp = prettier.format(pp, options);
+    if (pp !== pppp) {
+      const diff = require('diff').createTwoFilesPatch('', '', pp, pppp, '', '', {context: 2});
+      console.error(diff);
+    }
+    return;
+  }
+
   return prettier.format(input, options);
 }
 
@@ -171,7 +183,7 @@ if (stdin) {
 } else {
   eachFilename(filepatterns, filename => {
     fs.readFile(filename, "utf8", (err, input) => {
-      if (write) {
+      if (write || argv["debug-check"]) {
         console.log(filename);
       }
 

package.json

@@ -34,7 +34,8 @@
     "rollup-plugin-node-builtins": "2.0.0",
     "rollup-plugin-node-globals": "1.1.0",
     "rollup-plugin-node-resolve": "2.0.0",
-    "rollup-plugin-real-babili": "1.0.0-alpha3"
+    "rollup-plugin-real-babili": "1.0.0-alpha3",
+    "diff": "3.2.0"
   },
   "scripts": {
     "test": "jest",