Exclude "&" from the XML entity names

[calixteman]

Scanning to the end of the string for every "&" made the entity resolution quadratic. Stopping at the next "&" also fixes a bare ampersand swallowing the reference which follows it.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.99%. Comparing base (d0779c4) to head (f7f30dd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21692      +/-   ##
==========================================
- Coverage   90.00%   89.99%   -0.01%     
==========================================
  Files         263      263              
  Lines       66861    66861              
==========================================
- Hits        60178    60172       -6     
- Misses       6683     6689       +6     

Flag	Coverage Δ
browsertest	66.53% <100.00%> (+<0.01%)	⬆️
fonttest	9.04% <ø> (ø)
integrationtest	69.33% <100.00%> (-0.03%)	⬇️
unittest	57.86% <100.00%> (+0.03%)	⬆️
unittestcli	56.57% <100.00%> (+<0.01%)	⬆️

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[timvandermeij]

Thank you for improving this!