make: allow diff argument

E.g.:

    # make diff-dns/dyndns

Makefile

@@ -53,10 +53,7 @@ ${TARGET}:
 .  endfor
 .endfor
 
-diff:
-	@git diff --stat -p stable/${PLUGIN_ABI}
-
-ARGS=	mfc
+ARGS=	diff mfc
 
 # handle argument expansion for required targets
 .for TARGET in ${.TARGETS}
@@ -72,6 +69,9 @@ ${_TARGET}_ARG=		${${_TARGET}_ARGS:[0]}
 .endif
 .endfor
 
+diff:
+	@git diff --stat -p stable/${PLUGIN_ABI} ${.CURDIR}/${diff_ARGS:[1]}
+
 mfc:
 	@git checkout stable/${PLUGIN_ABI}
 .for MFC in ${mfc_ARGS}