tlog: only enable color if both stderr and stdout are a terminal

This

    gocryptfs -init /does/not/exist 2> err.log

used to write escape codes into err.log. Stop doing that.

internal/tlog/log.go

@@ -108,7 +108,9 @@ var Fatal *toggledLogger
 func init() {
 	// Enable color output if we are connected to a terminal and NO_COLOR is
 	// unset ( https://no-color.org/ ).
-	if _, nocolor := os.LookupEnv("NO_COLOR"); !nocolor && term.IsTerminal(int(os.Stdout.Fd())) {
+	if _, nocolor := os.LookupEnv("NO_COLOR"); !nocolor &&
+		term.IsTerminal(int(os.Stdout.Fd())) &&
+		term.IsTerminal(int(os.Stderr.Fd())) {
 		ColorReset = "\033[0m"
 		ColorGrey = "\033[2m"
 		ColorRed = "\033[31m"