Use trimmed length when --nth is used with --tiebreak=length

This change improves sort ordering for aligned tabular input.
Given the following input:

    apple   juice   100
    apple   pie     200

fzf --nth=2 will now prefer the one with pie. Before this change fzf
compared "juice   " and "pie     ", both of which have the same length.

src/util/util_test.go

@@ -20,3 +20,23 @@ func TestContrain(t *testing.T) {
 		t.Error("Expected", 3)
 	}
 }
+
+func TestTrimLen(t *testing.T) {
+	check := func(str string, exp int) {
+		trimmed := TrimLen([]rune(str))
+		if trimmed != exp {
+			t.Errorf("Invalid TrimLen result for '%s': %d (expected %d)",
+				str, trimmed, exp)
+		}
+	}
+	check("hello", 5)
+	check("hello ", 5)
+	check("hello  ", 5)
+	check(" hello", 5)
+	check("  hello", 5)
+	check(" hello ", 5)
+	check("  hello  ", 5)
+	check("h   o", 5)
+	check("  h   o  ", 5)
+	check("         ", 0)
+}