shallow.c: bit manipulation tweaks

First of all, 1 << 31 is technically undefined behaviour, so let's just
use an unsigned literal.

If i is 'signed int' and gcc doesn't know that i is positive, gcc
generates code to compute the C99-mandated values of "i / 32" and "i %
32", which is a lot more complicated than simple a simple shifts/mask.

The only caller of paint_down actually passes an "unsigned int" value,
but the prototype of paint_down causes (completely well-defined)
conversion to signed int, and gcc has no way of knowing that the
converted value is non-negative. Just make the id parameter unsigned.

In update_refstatus, the change in generated code is much smaller,
presumably because gcc is smart enough to see that i starts as 0 and is
only incremented, so it is allowed (per the UD of signed overflow) to
assume that i is always non-negative. But let's just help less smart
compilers generate good code anyway.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

shallow.c

@@ -389,7 +389,7 @@ static uint32_t *paint_alloc(struct paint_info *info)
  * all walked commits.
  */
 static void paint_down(struct paint_info *info, const unsigned char *sha1,
-		       int id)
+		       unsigned int id)
 {
 	unsigned int i, nr;
 	struct commit_list *head = NULL;
@@ -401,7 +401,7 @@ static void paint_down(struct paint_info *info, const unsigned char *sha1,
 	if (!c)
 		return;
 	memset(bitmap, 0, bitmap_size);
-	bitmap[id / 32] |= (1 << (id % 32));
+	bitmap[id / 32] |= (1U << (id % 32));
 	commit_list_insert(c, &head);
 	while (head) {
 		struct commit_list *p;
@@ -575,11 +575,11 @@ static int add_ref(const char *refname, const struct object_id *oid,
 
 static void update_refstatus(int *ref_status, int nr, uint32_t *bitmap)
 {
-	int i;
+	unsigned int i;
 	if (!ref_status)
 		return;
 	for (i = 0; i < nr; i++)
-		if (bitmap[i / 32] & (1 << (i % 32)))
+		if (bitmap[i / 32] & (1U << (i % 32)))
 			ref_status[i]++;
 }