Makefile: do not use sparse on third-party sources

We have several third-party sources in our codebase that we have
imported from upstream projects. These sources are mostly excluded from
our static analysis, for example when running Coccinelle.

Do the same for our "sparse" target by filtering them out.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

Makefile

@@ -3252,7 +3252,8 @@ t/helper/test-%$X: t/helper/test-%.o GIT-LDFLAGS $(GITLIBS) $(REFTABLE_TEST_LIB)
 check-sha1:: t/helper/test-tool$X
 	t/helper/test-sha1.sh
 
-SP_OBJ = $(patsubst %.o,%.sp,$(OBJECTS))
+SP_SRC = $(filter-out $(THIRD_PARTY_SOURCES),$(patsubst %.o,%.c,$(OBJECTS)))
+SP_OBJ = $(patsubst %.c,%.sp,$(SP_SRC))
 
 $(SP_OBJ): %.sp: %.c %.o $(GENERATED_H)
 	$(QUIET_SP)cgcc -no-compile $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) \