rust: fix signature of rustlazyancestors_init() function
Obviously, sizeof(int) != mem::size_of::<usize>() on amd64, though the
argument would be passed in 64-bit register anyway.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a