misleading output in `summary.txt` file
Hi,

We had an issue which might be interesting for you (and other users).
Whe have a WES sample, and after a bug on a storage system, we only get reads on the chr1 after doing an `intersectBed` of the `bam` with the `bed` target file. Nothing related to `mostdepth` so far.

However, when we compute the coverage of this bam file with ;
```
mosdepth -t 4 -n --quantize 0:1:10:50:100: --by Twist_Exome_Core_Covered_Targets_hg38.bed PREFIX onTarget.bam
```
It reports a very high total mean coverage (181.34X) in the summary file (`grep total_region`)

```
chrom	length	bases	mean	min	max
chr1	248956422	937505185	3.77	0	2887
chr1_region	3358574	609028999	181.34	0	2887
total	248956422	937505185	3.77	0	2887
total_region	3358574	609028999	181.34	0	2887
```

So to me this is misleading, as the `total_region` does not seem to take into account the full bed, but only the non-zero chromosomes. 

Of note, this is confirmed using the `regions.bed.gz` file but the 'real' mean coverage should be 18.36X.

```
>>zcat regions.bed.gz | awk '$1=="chr1"{ z=($3-$2); s=s+z*$4; c=c+z}END{print s/c}'
181.336
>>zcat test.regions.bed.gz | awk '{ z=($3-$2); s=s+z*$4; c=c+z}END{print s/c}'
18.3644
```

