feat: only save files if they are unchanged

Closes postcss#320

index.js

@@ -250,11 +250,11 @@ function css(css, file) {
           const tasks = []
 
           if (options.to) {
-            tasks.push(fs.outputFile(options.to, result.css))
+            tasks.push(outputFile(options.to, result.css))
 
             if (result.map) {
               const mapfile = getMapfile(options)
-              tasks.push(fs.outputFile(mapfile, result.map.toString()))
+              tasks.push(outputFile(mapfile, result.map.toString()))
             }
           } else process.stdout.write(result.css, 'utf8')
 
@@ -278,6 +278,13 @@ function css(css, file) {
     .catch((err) => {
       throw err
     })
+
+  async function outputFile(file, string) {
+    const fileExists = await fs.pathExists(file)
+    const currentValue = fileExists ? await fs.readFile(file, 'utf8') : null
+    if (currentValue === string) return
+    return fs.outputFile(file, string)
+  }
 }
 
 function dependencies(results) {

test/fixtures/unchanged-input.css

@@ -0,0 +1,3 @@
+body {
+  color: red;
+}

test/unchanged.js

@@ -0,0 +1,17 @@
+import fs from 'fs-extra'
+import test from 'ava'
+
+import cli from './helpers/cli.js'
+
+test('files are not saved if the contents are the same', async (t) => {
+  const input = 'test/fixtures/unchanged-input.css'
+  const output = 'test/fixtures/unchanged-output.css'
+  const intialStat = await fs.stat(output)
+
+  const { error, stderr } = await cli([input, '-o', output])
+
+  t.falsy(error, stderr)
+
+  const finalStat = await fs.stat(output)
+  t.is(finalStat.mtimeMs, intialStat.mtimeMs)
+})