cli/tree: Use single character triple dot

Using 3 characters instead of 1 to ellipsize a long string is wasteful.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

cli/command/image/tree.go

@@ -309,7 +309,7 @@ type imgColumn struct {
 func truncateRunes(s string, length int) string {
 	runes := []rune(s)
 	if len(runes) > length {
-		return string(runes[:length-3]) + "..."
+		return string(runes[:length-1]) + "…"
 	}
 	return s
 }