Avoid modifying correctly-formatted files

Currently, when running with `--mode=inplace`, Ormolu always overwrites the
files, even when nothing has changed, updating the modified timestamp of the
file. This encourages Stack to rebuild everything when only one file has
changed.

A quick check to see if the file needs rewriting solves this problem.

I would prefer the API allowed me to only read the file once, but as the
`--mode=check` logic also reads the file twice, I decided this wasn't the
biggest sin for now.

CHANGELOG.md

@@ -1,3 +1,8 @@
+## Unreleased
+
+* Ormolu no longer overwrites already formatted files. [PR
+  649](https://github.com/tweag/ormolu/pull/649).
+
 ## Ormolu 0.1.2.0
 
 * Fixed the bug when comments in different styles got glued together after

app/Main.hs

@@ -61,15 +61,19 @@ formatOne mode config = \case
         -- 101 is different from all the other exit codes we already use.
         exitWith (ExitFailure 101)
   Just inputFile -> do
-    r <- ormoluFile config inputFile
+    originalInput <- TIO.readFile inputFile
+    formattedInput <- ormoluFile config inputFile
     case mode of
       Stdout ->
-        TIO.putStr r
-      InPlace ->
-        TIO.writeFile inputFile r
+        TIO.putStr formattedInput
+      InPlace -> do
+        -- Only write when the contents have changed, in order to avoid
+        -- updating the modified timestamp if the file was already correctly
+        -- formatted.
+        when (formattedInput /= originalInput) $
+          TIO.writeFile inputFile formattedInput
       Check -> do
-        r' <- TIO.readFile inputFile
-        when (r /= r') $
+        when (formattedInput /= originalInput) $
           -- 100 is different to all the other exit code that are emitted
           -- either from an 'OrmoluException' or from 'error' and
           -- 'notImplemented'.