HTSlib does not support VCF v4.4's explicit first phasing indicator

[jmarshall]

This VCF file includes some explicit first phasing indicators, as defined in VCF v4.4 and v4.5's §1.6.2:

##fileformat=VCFv4.4
##FILTER=<ID=PASS,Description="All filters passed">
##contig=<ID=chr1,length=60>
##FORMAT=<ID=GT,Number=1,Type=String,Description="Genotype">
#CHROM	POS	ID	REF	ALT	QUAL	FILTER	INFO	FORMAT	s1	s2
chr1	10	.	A	T,C	60	PASS	.	GT	0/1/2	|0/1/2
chr1	20	.	A	T,C	60	PASS	.	GT	/0|1|2	|0/1/2
chr1	30	.	G	C,A	60	PASS	.	GT	0|1|2	0|1/2

HTSlib fails to parse this file, with the following complaint:

$ htsfile -c genotype.vcf    # or similarly with bcftools view
[E::vcf_parse_format_fill5] Couldn't read GT data: value not a number or '.' at chr1:10

The record at position 10 contains the example genotype from the spec, with and without an explicit first phasing indicator. That at position 20 explicitly sets first-phasing that differs from the implicit value if the indicators were omitted. And the record at position 30 has genotypes with the usual implicit first-phasing that in VCFv4.4 would AIUI correspond to the explicit |0|1|2 and /0|1/2 respectively.

(As previously noted at sgkit-dev/bio2zarr#263. See also #1113, which is tangentially related.)