compat: introduce on_disk_bytes()

Some platforms do not have st_blocks member in "struct stat"; mingw
already emulates it by rounding it up to closest 512-byte blocks (even
though it could overcount when a file has holes).

The reason to use the member is only to figure out how many kilobytes the
files occupy on-disk, so give a helper function in git-compat-util.h to
compute this value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Johannes Sixt <johannes.sixt@telecom.at>

compat/mingw.h

@@ -169,7 +169,6 @@ struct mingw_stat {
 	time_t st_mtime, st_atime, st_ctime;
 	unsigned st_dev, st_ino, st_uid, st_gid;
 	size_t st_size;
-	size_t st_blocks;
 };
 int mingw_lstat(const char *file_name, struct mingw_stat *buf);
 int mingw_fstat(int fd, struct mingw_stat *buf);