tree-walk.c: make tree_entry_interesting() take an index

tree-walk.c: make tree_entry_interesting() take an index

In order to support :(attr) when matching pathspec on a tree,
tree_entry_interesting() needs to take an index (because
git_check_attr() needs it). This is the preparation step for it. This
also makes it clearer what index we fall back to when looking up
attributes during an unpack-trees operation: the source index.

This also fixes revs->pruning.repo initialization that should have
been done in 2abf350385 (revision.c: remove implicit dependency on
the_index - 2018-09-21). Without it, skip_uninteresting() will
dereference a NULL pointer through this call chain

  get_revision(revs)
  get_revision_internal
  get_revision_1
  try_to_simplify_commit
  rev_compare_tree
  diff_tree_oid(..., &revs->pruning)
  ll_diff_tree_oid
  diff_tree_paths
  ll_diff_tree
  skip_uninteresting

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