tree-walk.h: fix incorrect API comment

tree-walk.h: fix incorrect API comment

When commit 50ddb089ff68 (tree-walk.c: remove the_repo from
get_tree_entry(), 2019-06-27) added an extra parameter to
get_tree_entry(), it did not fix the ordering comment about the meaning
of the parameters.  Rather than just changing "third"->"fourth" and
"fourth"->"fifth", give the paramemters meaningful names (or actually,
just take the existing names from the get_tree_entry() definition in the
tree-walk.c file) and while at it, tweak the rest of the description to
incorporate the other parameter names as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>