command/fmt: Factor out the actual formatting

Previously formatting was just a simple wrapper around hclwrite.Format.
That remains true here, but the call is factored out into a separate
method in preparation for making it also do some Terraform-specific
cleanups in a future commit.

command/fmt.go

@@ -176,7 +176,7 @@ func (c *FmtCommand) processFile(path string, r io.Reader, w io.Writer, isStdout
 		return diags
 	}
 
-	result := hclwrite.Format(src)
+	result := c.formatSourceCode(src)
 
 	if !bytes.Equal(src, result) {
 		// Something was changed
@@ -266,6 +266,12 @@ func (c *FmtCommand) processDir(path string, stdout io.Writer) tfdiags.Diagnosti
 	return diags
 }
 
+// formatSourceCode is the formatting logic itself, applied to each file that
+// is selected (directly or indirectly) on the command line.
+func (c *FmtCommand) formatSourceCode(src []byte) []byte {
+	return hclwrite.Format(src)
+}
+
 func (c *FmtCommand) Help() string {
 	helpText := `
 Usage: terraform fmt [options] [DIR]