execute: Open separate handles to /dev/tty (in, out, err)

# This will no longer cause 'Vim: Warning: Output is not to a terminal'
  fzf --bind 'enter:execute:vim {}' > /tmp/foo

src/reader.go

@@ -113,7 +113,7 @@ func (r *Reader) ReadSource(inputChan chan string, root string, opts walkerOpts,
 	var success bool
 	if inputChan != nil {
 		success = r.readChannel(inputChan)
-	} else if util.IsTty() {
+	} else if util.IsTty(os.Stdin) {
 		cmd := os.Getenv("FZF_DEFAULT_COMMAND")
 		if len(cmd) == 0 {
 			success = r.readFiles(root, opts, ignores)