zsh: eagerly define diff() to avoid prezto autoload-stub error

Pretzo's modules/utility/functions/diff registers an autoload stub that
emits '(eval):1: diff: function definition file not found' in non-
interactive eval contexts where $fpath doesn't include the utility
module's functions directory. Define diff as a real function up front so
the stub never has to look itself up.

zsh/diff.zsh

@@ -0,0 +1,13 @@
+# Eagerly define `diff` as a real function instead of relying on prezto's
+# autoload stub. The autoload stub emits
+#   "(eval):1: diff: function definition file not found"
+# in non-interactive eval contexts where $fpath doesn't yet include the
+# prezto utility module's functions directory. Defining a real function
+# here bypasses the autoload path entirely.
+function diff {
+  if (( $+commands[colordiff] )); then
+    command diff --unified "$@" | colordiff --difftype diffu
+  else
+    command diff --unified "$@"
+  fi
+}