SV validate: improve selection of merge sizes
SV validate: improve selection of merge sizes

Avoid having no overlap when truth set contains lots of small regions, like
the GiaB NA24385 validation set

vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen