index-pack, unpack-objects: use size_t for object size

When unpacking objects from a packfile, the object size is decoded
from a variable-length encoding. On platforms where unsigned long is
32-bit (such as Windows, even in 64-bit builds), the shift operation
overflows when decoding sizes larger than 4GB. The result is a
truncated size value, causing the unpacked object to be corrupted or
rejected.

Fix this by changing the size variable to size_t, which is 64-bit on
64-bit platforms, and ensuring the shift arithmetic occurs in 64-bit
space.

Declare the per-byte continuation variable `c` as size_t as well,
matching the canonical varint decoder unpack_object_header_buffer()
in packfile.c. With c as size_t the expression (c & 0x7f) << shift
is naturally size_t-typed, so the explicit cast that an earlier
iteration carried at the use site is no longer needed.

While at it, add the same overflow guard that
unpack_object_header_buffer() carries: if the cumulative shift would
exceed bitsizeof(size_t) - 7, refuse the input rather than invoking
undefined behavior. Unlike unpack_object_header_buffer(), which
labels this case "bad object header", report it as the platform
limit it actually is: a header may be perfectly well-formed and
still encode a size we cannot represent locally (notably on a
32-bit build consuming a packfile produced on a 64-bit host).

This was originally authored by LordKiRon <https://github.com/LordKiRon>,
who preferred not to reveal their real name and therefore agreed that I
take over authorship.

Helped-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/unpack-objects.c

@@ -533,7 +533,7 @@ static void unpack_one(unsigned nr)
 {
 	unsigned shift;
 	unsigned char *pack;
-	unsigned long size, c;
+	size_t size, c;
 	enum object_type type;
 
 	obj_list[nr].offset = consumed_bytes;
@@ -545,6 +545,8 @@ static void unpack_one(unsigned nr)
 	size = (c & 15);
 	shift = 4;
 	while (c & 0x80) {
+		if ((bitsizeof(size_t) - 7) < shift)
+			die(_("object size too large for this platform"));
 		pack = fill(1);
 		c = *pack;
 		use(1);