Remove additions from contrib/ before moving HEAD

* mk/targets.mk (up0): Explicitly remove additions from contrib/
  before moving HEAD via git since the files might be renamed, deleted
  or moved by checking out a new version.

Thanks to Suvayu Ali for pointing to this problem.  The user still has
to remember to do this when moving HEAD manually, though.

mk/targets.mk

@@ -102,6 +102,7 @@ ifeq ($(TEST_NO_AUTOCLEAN),) # define this variable to leave $(testdir) around f
 	$(MAKE) cleantest
 endif
 
+up0::	cleanaddcontrib
 up0 up1 up2::
 	git remote update
 	git pull