dir: expose cmp_name() and check_contains()

dir: expose cmp_name() and check_contains()

We want to use cmp_name() and check_contains() (which both compare
`struct dir_entry`s, the former in terms of the sort order, the latter
in terms of whether one lexically contains another) outside of dir.c,
so we have to (1) change their linkage and (2) rename them as
appropriate for the global namespace. The second is achieved by
renaming cmp_name() to cmp_dir_entry() and check_contains() to
check_dir_entry_contains().

Signed-off-by: Samuel Lijin <sxlijin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>