Fix fetching of local metrics.

If a given metric matched two queries (for example, node.cpu.user
matches a query for both "node.cpu.user" and "node.cpu.*") and that
metric was on the local filesystem (that is, not fetched from another
Graphite in a cluster), we were mishandling that metric. The
consequence was that a render request for N targets could get more
than N results back, which either made a graph that confused humans or
emitted JSON that confused flot.