Cast 64 bit off_t to 32 bit size_t

Some systems have sizeof(off_t) == 8 while sizeof(size_t) == 4.
This implies that we are able to access and work on files whose
maximum length is around 2^63-1 bytes, but we can only malloc or
mmap somewhat less than 2^32-1 bytes of memory.

On such a system an implicit conversion of off_t to size_t can cause
the size_t to wrap, resulting in unexpected and exciting behavior.
Right now we are working around all gcc warnings generated by the
-Wshorten-64-to-32 option by passing the off_t through xsize_t().

In the future we should make xsize_t on such problematic platforms
detect the wrapping and die if such a file is accessed.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

combine-diff.c

@@ -684,7 +684,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 			goto deleted_file;
 
 		if (S_ISLNK(st.st_mode)) {
-			size_t len = st.st_size;
+			size_t len = xsize_t(st.st_size);
 			result_size = len;
 			result = xmalloc(len + 1);
 			if (result_size != readlink(elem->path, result, len)) {
@@ -697,7 +697,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 		}
 		else if (0 <= (fd = open(elem->path, O_RDONLY)) &&
 			 !fstat(fd, &st)) {
-			size_t len = st.st_size;
+			size_t len = xsize_t(st.st_size);
 			size_t sz = 0;
 			int is_file, i;