fix(theme-and-appearance): fix diff completion in macOS

Closes #11416
Closes #11454

lib/theme-and-appearance.zsh

@@ -41,7 +41,11 @@ fi
 
 # enable diff color if possible.
 if command diff --color /dev/null /dev/null &>/dev/null; then
-  alias diff='diff --color'
+  function color-diff {
+    diff --color $@
+  }
+  alias diff="color-diff"
+  compdef _diff color-diff # compdef is already loaded by this point
 fi
 
 setopt auto_cd