per-readgroup depth not working in v0.2.5
I'm having trouble extracting particular readgroups from a bam, using version 0.2.5 (running via the quay.io docker container - `quay.io/biocontainers/mosdepth:0.2.5--hb763d49_0`)

If I run the tool on a small test bam without specifying readgroups, I get the expected results:

```
$ mosdepth -b 10000 -n test -f all_sequences.fa new.bam

$ gunzip -c test.regions.bed.gz | head -n 5
chr1    0       10000   0.01
chr1    10000   20000   0.26
chr1    20000   30000   0.00
chr1    30000   40000   0.00
chr1    40000   50000   0.00
```
If I run the command on the same bam, including one or both readgroups, I get all zeroes for output:

```
$ mosdepth -b 10000 --read-groups 2900978675 -n test -f all_sequences.fa new.bam

$ gunzip -c test.regions.bed.gz | head -n 5
chr1    0       10000   0.00
chr1    10000   20000   0.00
chr1    20000   30000   0.00
chr1    30000   40000   0.00
chr1    40000   50000   0.00

```

Double checking to see that I got the readgroups correct. In the header:

```
@RG   ID:2900972070  CN:WUGSC LB:TWDS-080075-1135-M1512746_1-lg1-lib1   PL:Illumina PU:HH33GDSXX.2.GTCCTTCT-CTGACACA SM:TWDS-080075-1135-M1512746_1
@RG   ID:2900978675  CN:WUGSC LB:TWDS-080075-1135-M1512746_1-lib1 PL:Illumina PU:HLFY3DSXX.4.GTCCTTCT-CTGACACA SM:TWDS-080075-1135-M1512746_1

```
And both readgroups are represented in reads:

```
A00118:130:HLFY3DSXX:4:1242:3893:15060 83 chr1  9996  0  87S64M   =  10010 -50   CCCCCCCCCCCCCCCCCCCCTCACCATTTCAAGAAGAATACGGCATACGATATAGATGTACGTTTATGTTTTTCTTACGTTTGCTCTTCCGATATCCCTAACCCTAACCCTAACCCTAACCCTAACCCTAACCCTAACCCTAACCCTAACC  ??+??????+????+??+?++++?++?++??????++?'+&+5++??&??+????++?+?'?++++??+++??5+++?+?+?5?????+????+?????????????????????????????????????????????????????????  MC:Z:56M95S PG:Z:MarkDuplicates  MQ:i:0   AS:i:59  XS:i:56  MD:Z:0N0N0N0N0N2A56  NM:i:6   RG:Z:2900978675

A00584:86:HH33GDSXX:2:2335:10863:1078  2147  chr1  10055 0  6M1D64M81S  chr11 62535917 0  TAACCCAACCCTAACCCTAACCCTAACCCTAACCCTAACCCTAACCCTAACCCAACCCTAACCCTAACCCAGTCCACACCCTGGGGTGACTCACCCTCCTTGACCACCCCAGTGCGGGCCGCAGGGGAGTTCTGCGTCACCTCCTGCACAA  ?????????????????????????????????????????5???????????????????+????++???5????????????????????????5??5????????????+????????????????????????????????5?????  SA:Z:chr11,62535917,+,66S85M,60,0;  MC:Z:61S90M PG:Z:MarkDuplicates  MQ:i:41  AS:i:64  XS:i:63  MD:Z:6^T64  NM:i:1   RG:Z:2900972070

```

Any insight into what's going on here?

I posted this tiny example bam and fastq here:
https://xfer.genome.wustl.edu/gxfer1/project/cancer-genomics/mosdepth/

Thanks!