snp_filter: Pre-increment pivot during right extension
snp_filter: Pre-increment pivot during right extension

The base pair at the starting point was being consumed during both left
and right extension.

Modify the right extension loop to pre-increment the pivot index. This
makes it so that left extension consumes the starting point, but right
extension does not.

broadinstitute/firepony,broadinstitute/firepony,broadinstitute/firepony,NVlabs/firepony,broadinstitute/firepony,NVlabs/firepony,NVlabs/firepony,NVlabs/firepony