midx repack: avoid integer overflow on 32 bit systems

On a 32 bit system "git multi-pack-index --repack --batch-size=120M"
failed with

    fatal: size_t overflow: 6038786 * 1289

The calculation to estimated size of the objects in the pack referenced
by the multi-pack-index uses st_mult() to multiply the pack size by the
number of referenced objects before dividing by the total number of
objects in the pack. As size_t is 32 bits on 32 bit systems this
calculation easily overflows. Fix this by using 64bit arithmetic instead.

Also fix a potential overflow when caluculating the total size of the
objects referenced by the multipack index with a batch size larger
than SIZE_MAX / 2. In that case

    total_size += estimated_size

can overflow as both total_size and estimated_size can be greater that
SIZE_MAX / 2. This is addressed by using saturating arithmetic for the
addition. Although estimated_size is of type uint64_t by the time we
reach this sum it is bounded by the batch size which is of type size_t
and so casting estimated_size to size_t does not truncate the value.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -668,6 +668,22 @@ static inline int cast_size_t_to_int(size_t a)
 	return (int)a;
 }
 
+static inline uint64_t u64_mult(uint64_t a, uint64_t b)
+{
+	if (unsigned_mult_overflows(a, b))
+		die("uint64_t overflow: %"PRIuMAX" * %"PRIuMAX,
+		    (uintmax_t)a, (uintmax_t)b);
+	return a * b;
+}
+
+static inline uint64_t u64_add(uint64_t a, uint64_t b)
+{
+	if (unsigned_add_overflows(a, b))
+		die("uint64_t overflow: %"PRIuMAX" + %"PRIuMAX,
+		    (uintmax_t)a, (uintmax_t)b);
+	return a + b;
+}
+
 /*
  * Limit size of IO chunks, because huge chunks only cause pain.  OS X
  * 64-bit is buggy, returning EINVAL if len >= INT_MAX; and even in