Make xgettext respect granularity for sub-chapter messages

[carreter]

Related to #171.

While trying to refresh the Spanish translation of Comprehensive Rust, @henrif75 noticed that the messages.pot file I generated had line numbers that shouldn't be there (see google/comprehensive-rust#2120).

Digging around in the code, I noticed that the xgettext code was ignoring the granularity for sub-chapters. This PR fixes that!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.55172% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.02%. Comparing base (68d2e29) to head (5cddc82).
Report is 6 commits behind head on main.

Files	Patch %	Lines
i18n-helpers/src/xgettext.rs	96.55%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
- Coverage   86.99%   85.02%   -1.98%     
==========================================
  Files          15       15              
  Lines        3229     3398     +169     
  Branches     3229     3398     +169     
==========================================
+ Hits         2809     2889      +80     
- Misses        327      413      +86     
- Partials       93       96       +3     

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[kdarkhan]

Thanks for the fix, @carreter! Can you add or update one of the existing test cases which will catch this bug please?

I think it will be good to have this tested to ensure we don't break it in the future.

[carreter]

Added the test as requested @kdarkhan !

[kdarkhan]

Please fix formatting. LGTM otherwise.

[kdarkhan]

I added the fix on top of your PR, thanks for the contribution.