Teach tree_entry_interesting() that the tree entries are sorted.

Teach tree_entry_interesting() that the tree entries are sorted.

When we are looking at a tree entry with pathspecs, if all the
pathspecs sort strictly earlier than the entry we are currently
looking at, there is no way later entries in the same tree would
match our pathspecs, because the entries are sorted.

Signed-off-by: Junio C Hamano <junkio@cox.net>