Fix multiple message comments in nested calls

[bkryza]

Fixes #301

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 88.05970% with 8 lines in your changes missing coverage. Please review.

Project coverage is 90.49%. Comparing base (7df74e7) to head (34e1012).

Files	Patch %	Lines
src/common/generators/plantuml/generator.h	16.66%	5 Missing ⚠️
src/common/visitor/ast_id_mapper.cc	85.71%	1 Missing ⚠️
src/sequence_diagram/model/message.cc	94.11%	1 Missing ⚠️
...quence_diagram/visitor/translation_unit_visitor.cc	95.65%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #304      +/-   ##
==========================================
+ Coverage   90.47%   90.49%   +0.01%     
==========================================
  Files         123      123              
  Lines       13801    13803       +2     
==========================================
+ Hits        12487    12491       +4     
+ Misses       1314     1312       -2     

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