Document broken branch merging logic

[Swatinem]

This is coming from a customer issue.
The customer has an upload that contains both an lcov file as well as a cobertura file.

The lcov file creates a LineSession with 0/2 coverage and missed_branches=[0:5, 0:6].
The cobertura file encodes the same information in a different format, yielding 0/1 with missed_braches=[0].

Merging those two is currently broken and yields a bogus result of 2/2.

[codecov-notifications]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.58%. Comparing base (fd909cc) to head (360a256).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #585   +/-   ##
=======================================
  Coverage   88.58%   88.58%           
=======================================
  Files         462      462           
  Lines       12873    12873           
  Branches     1461     1461           
=======================================
  Hits        11403    11403           
  Misses       1165     1165           
  Partials      305      305           

Flag	Coverage Δ
shared-docker-uploader	88.58% <100.00%> (ø)

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[codspeed-hq]

CodSpeed Performance Report

Merging #585 will not alter performance

_{Comparing swatinem/broken-branchmerge (360a256) with main (fd909cc)}

Summary

✅ 9 untouched benchmarks