zero plugin zeroes disctotal if single disc

[fredrik]

Description

When omit_single_disc is set, disctotal is now also zeroed alongside disc. These tags work together ("Disc 2 of 3") so keeping one without the other is inconsistent.

Previously, only the disc tag was zeroed. This follows from #6015 which made it into v2.5.1 and added the omit_single_disc option.

I've added tests and have used this locally for some time. Happy to add documentation (zero.rst) and changelog updates if maintainers think it's warranted.

To Do

• Tests
• Documentation (pending maintainer input)
• Changelog (pending maintainer input)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.71%. Comparing base (ef59cfa) to head (4165213).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/zero.py	66.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6306      +/-   ##
==========================================
- Coverage   68.71%   68.71%   -0.01%     
==========================================
  Files         138      138              
  Lines       18558    18562       +4     
  Branches     3062     3063       +1     
==========================================
+ Hits        12753    12755       +2     
  Misses       5154     5154              
- Partials      651      653       +2     

Files with missing lines	Coverage Δ
beetsplug/zero.py	92.30% <66.66%> (-1.95%)	⬇️

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