If a pagespec fails to match, I had been throwing the influences away, but
that is not right. Consider `backlink(foo)`, where foo does not exist.
It still needs to be added as an influence, because if it is created, it
will influence the pagespec to match.
But with that fix, `link(bar)` had as influences all pages, whether they
link to bar or not. Which is not necessary, because modifiying a page to
add a link to bar will directly cause the pagespec to match.
So, in match_link (and all the match_* functions for page metadata),
only return an influence if the match succeeds.
match_backlink had been implemented as the inverse of match_link, but that
is no longer completly true. While match_link does not return an influence
on failure, match_backlink does.
match_created_before/after also return the influence on failure, this way
if created_after(foo) currently fails because foo does not exist, it will
still update the page with the pagespec if foo is created.