zsh:diffcmds: Add completion function

Completes as the first argument command names and uses then their
completion function to complete the rest.

TODO: <COMMENT>

.config/zsh/completion/_diffcmds

@@ -0,0 +1,14 @@
+#compdef diffcmds
+
+# Very much simplified version of _sudo
+
+local -a args
+
+# TODO: complete behind `--` whatever would have been completed at the position
+# of `%%`
+args+=(
+	"(-)1: :{ _command_names -e }"
+	"*:: :{ _normal }"
+)
+
+_arguments $args