tree-walk.c: ignore trailing slash on submodule in tree_entry_interesting()

We do ignore trailing slash on a directory, so pathspec "abc/" matches
directory "abc". A submodule is also a directory. Apply the same logic
to it. This makes "git log submodule-path" and "git log submodule-path/"
produce the same output.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>