improve germline filtering: handle multiple ALTs better

[Rohan Vanheusden]

I was just thinking—right now, the germline filter will only match two records if the CHROM, POS, REF, and ALT are all the same. But, since VCFs will encode all mutations that happen at a particular POS as multiple ALTs, we could instead just check if one (not all) of the ALTs match, and then remove that ALT specifically.

[Rohan Vanheusden]

Implemented by 63bdeeb.