hclfmt: avoid rewrites when there are no changes in a file

this is faster for large trees and also helps to make tools like
[treefmt](https://github.com/numtide/treefmt) to work.

cmd/hclfmt/main.go

@@ -106,6 +106,7 @@ func processFiles() error {
 
 func processFile(fn string, in *os.File) error {
 	var err error
+	var hasLocalChanges bool = false
 	if in == nil {
 		in, err = os.Open(fn)
 		if err != nil {
@@ -131,9 +132,10 @@ func processFile(fn string, in *os.File) error {
 
 	if !bytes.Equal(inSrc, outSrc) {
 		changed = append(changed, fn)
+		hasLocalChanges = true
 	}
 
-	if *overwrite {
+	if hashLocalChanges && *overwrite {
 		return ioutil.WriteFile(fn, outSrc, 0644)
 	}