Makefile: help people who run 'make check' by mistake

The target to run self test is 'make test', but there are people who try
'make check' and worse yet do not have sparse installed.

Suggest 'make test' target when they do not have 'sparse'.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1329,7 +1329,16 @@ check-sha1:: test-sha1$X
 	./test-sha1.sh
 
 check: common-cmds.h
-	for i in *.c; do sparse $(ALL_CFLAGS) $(SPARSE_FLAGS) $$i || exit; done
+	if sparse; \
+	then \
+		for i in *.c; \
+		do \
+			sparse $(ALL_CFLAGS) $(SPARSE_FLAGS) $$i || exit; \
+		done; \
+	else \
+		echo 2>&1 "Did you mean 'make test'?"; \
+		exit 1; \
+	fi
 
 remove-dashes:
 	./fixup-builtins $(BUILT_INS) $(PROGRAMS) $(SCRIPTS)