arbitraryfilecontext: skip the cmp fast path if any side is a symlink `filecmp` follows symlinks by default, which a `filectx.cmp()` call should not be doing as it should only compare the requested entry. After this patch, only the contexts' data are compared, which is the correct contract. This is a corrected version of D1122. Differential Revision: https://phab.mercurial-scm.org/D1165