commit-slab.h: avoid -Wsign-compare warnings

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-slab.h

@@ -78,7 +78,7 @@ static MAYBE_UNUSED void init_ ##slabname(struct slabname *s)		\
 									\
 static MAYBE_UNUSED void clear_ ##slabname(struct slabname *s)		\
 {									\
-	int i;								\
+	unsigned int i;							\
 	for (i = 0; i < s->slab_count; i++)				\
 		free(s->slab[i]);					\
 	s->slab_count = 0;						\
@@ -89,13 +89,13 @@ static MAYBE_UNUSED elemtype *slabname## _at_peek(struct slabname *s,	\
 						  const struct commit *c, \
 						  int add_if_missing)   \
 {									\
-	int nth_slab, nth_slot;						\
+	unsigned int nth_slab, nth_slot;				\
 									\
 	nth_slab = c->index / s->slab_size;				\
 	nth_slot = c->index % s->slab_size;				\
 									\
 	if (s->slab_count <= nth_slab) {				\
-		int i;							\
+		unsigned int i;						\
 		if (!add_if_missing)					\
 			return NULL;					\
 		REALLOC_ARRAY(s->slab, nth_slab + 1);			\