tree-walk.c: fix overoptimistic inclusion in :(exclude) matching

tree-walk.c: fix overoptimistic inclusion in :(exclude) matching

tree_entry_interesting() is used for matching pathspec on a tree. The
interesting thing about this function is that, because the tree
entries are known to be sorted, this function can return more than
just "yes, matched" and "no, not matched". It can also say "yes, this
entry is matched and so is the remaining entries in the tree".

This is where I made a mistake when matching exclude pathspec. For
exclude pathspec, we do matching twice, one with positive patterns and
one with negative ones, then a rule table is applied to determine the
final "include or exclude" result. Note that "matched" does not
necessarily mean include. For negative patterns, "matched" means
exclude.

This particular rule is too eager to include everything. Rule 8 says
that "if all entries are positively matched" and the current entry is
not negatively matched (i.e. not excluded), then all entries are
positively matched and therefore included. But this is not true. If
the _current_ entry is not negatively matched, it does not mean the
next one will not be and we cannot conclude right away that all
remaining entries are positively matched and can be included.

Rules 8 and 18 are now updated to be less eager. We conclude that the
current entry is positively matched and included. But we say nothing
about remaining entries. tree_entry_interesting() will be called again
for those entries where we will determine entries individually.

Reported-by: Christophe Bliard <christophe.bliard@trux.info>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>