utility module: fix/improve colordiff call in diff

The new call is compatible with colordiff v1.0.8 - v.1.0.15 (might be
compatible with even earlier versions, but not tested), while the
original one breaks down under v1.0.14 and v1.0.15. See
https://github.com/daveewart/colordiff/issues/22.

modules/utility/functions/diff

@@ -8,7 +8,7 @@
 function diff {
   if zstyle -t ':prezto:module:utility:diff' color; then
     if (( $+commands[colordiff] )); then
-      command diff --unified "$@" | colordiff --difftype diffu
+      command colordiff --unified "$@"
     elif (( $+commands[git] )); then
       git --no-pager diff --color=auto --no-ext-diff --no-index "$@"
     else