Add new variant type VCF_OVERLAP

[pd3]

See also
samtools/bcftools#736
samtools/bcftools#817
samtools/bcftools@db2e2b6

[daviesrob]

This needs commit samtools/bcftools@9208dda to be applied to bcftools to make the example in samtools/bcftools#817 work properly (without it you get nothing in the TYPE column for the spanning deletions). Would you like me to apply a rebased version of it to bcftools at the same time as merging this?

[pd3]

That would be great, thank you

[daviesrob]

Merged and rebased bcftools update pushed. You should be able to get rid of the bcftools vcf_overlap branch now.

[pd3]

Thanks!