cmd/goimports: set correct permissions on Windows

As of Go 1.14, WriteFile on Windows will set read-only on existing files
if you pass 0 for perms. Pass the pre-existing permissions.

Updates golang/go#38225.

Change-Id: I3174469efd4dc4c7eacc8522386a0712cfa39d11
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229297
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cmd/goimports/goimports.go

@@ -160,7 +160,12 @@ func processFile(filename string, in io.Reader, out io.Writer, argType argumentT
 				// filename is "<standard input>"
 				return errors.New("can't use -w on stdin")
 			}
-			err = ioutil.WriteFile(filename, res, 0)
+			// On Windows, we need to re-set the permissions from the file. See golang/go#38225.
+			var perms os.FileMode
+			if fi, err := os.Stat(filename); err == nil {
+				perms = fi.Mode() & os.ModePerm
+			}
+			err = ioutil.WriteFile(filename, res, perms)
 			if err != nil {
 				return err
 			}