When using CatVariants, VCF files were being sorted solely on the base
pair position of the first record, ignoring the chromosome. This can
become problematic when merging files from different chromosomes,
espeically if you have multiple VCFs per chromosome.
As an example, assume the following 3 lines are all in separate files:
1 10
1 100
2 20
The merged VCF from CatVariants (without -assumeSorted) would read:
1 10
2 20
1 100
This has the potential to break tools that expect chromosomes to be
contiguous within a VCF file.
This commit changes the comparator from one of Pair<Integer, File> to
one of Pair<VariantContext, File>. We construct a
VariantContextComparator from the provided reference, which will sort
the first record by chromosome and position properly. Additionally, if
-assumeSorted is given, we simply use a null VariantContext as the first
record, which will all be equal (as all will be null)
8fc631b7ae