influence blocker implementation

This avoids unnecessary influences being recorded from pagespecs
such as "link(done) and bugs/*", when a page cannot ever possibly
match.

A pagespec term that returns a value without influence is an influence
blocker. If such a blocker has a false value (possibly due to being
negated) and is ANDed with another term, it blocks that term's influence
from propigating out.

If the term is ORed, or has a true value, it does not block influence.
(Consider "link(done) or bugs/*" and "link(done) and !nosuchpage")

In the implementation in merge_influence, I had to be careful to never
negate $this or $other when testing if they are an influence blocker,
since negation mutates the object. Thus the slightly weird if statement.

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