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, since it does not
update the mtime of the file.

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 {
@@ -131,10 +132,15 @@ func processFile(fn string, in *os.File) error {
 
  if !bytes.Equal(inSrc, outSrc) {
  changed = append(changed, fn)
+ hasLocalChanges = true
  }
 
  if *overwrite {
- return ioutil.WriteFile(fn, outSrc, 0644)
+ if hasLocalChanges {
+ return ioutil.WriteFile(fn, outSrc, 0644)
+ } else {
+ return nil
+ }
  }
 
  _, err = os.Stdout.Write(outSrc)