Makefile: always remove .depend directories on 'make clean'

Even if COMPUTE_HEADER_DEPENDENCIES is not set, some .o.d files
might be lying around from previous builds when it was.  This
is especially likely because using the CHECK_HEADER_DEPENDENCIES
feature requires building sometimes with COMPUTE... on and
sometimes with it off.  At the end of such an exercise, to get
a blank slate, the user ought to be able to just run 'make clean'.
Make it so.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1693,9 +1693,9 @@ XDIFF_OBJS = xdiff/xdiffi.o xdiff/xprepare.o xdiff/xutils.o xdiff/xemit.o \
 OBJECTS := $(GIT_OBJS) $(XDIFF_OBJS)
 
 dep_files := $(foreach f,$(OBJECTS),$(dir $f).depend/$(notdir $f).d)
+dep_dirs := $(addsuffix .depend,$(sort $(dir $(OBJECTS))))
 
 ifdef COMPUTE_HEADER_DEPENDENCIES
-dep_dirs := $(addsuffix .depend,$(sort $(dir $(OBJECTS))))
 $(dep_dirs):
 	mkdir -p $@