tree_entry_interesting(): allow it to say "everything is interesting"

In addition to optimizing pathspecs that would never match,
which was done earlier, this optimizes pathspecs that would
always match (e.g. "arch/" while the traversal is already in
"arch/i386/" hierarchy).

This patch makes the worst case slightly more palatable, while
improving average case.

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