Makefile: do not allow gnu make to remove test-*.o files

It appears parallel build (-j) gets confused.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -969,6 +969,8 @@ test-date$X: date.o ctype.o
 
 test-delta$X: diff-delta.o patch-delta.o
 
+.PRECIOUS: $(patsubst test-%$X,test-%.o,$(TEST_PROGRAMS))
+
 test-%$X: test-%.o $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(LIBS)