Store last modified date and tagged releases in .gitmodules

[jwodder]

Part of #64.

[codecov]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.

Project coverage is 75.08%. Comparing base (25bbfa5) to head (0c02dfe).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/backups2datalad/datasetter.py	82.14%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   74.39%   75.08%   +0.69%     
==========================================
  Files          17       17              
  Lines        2816     2830      +14     
  Branches      403      405       +2     
==========================================
+ Hits         2095     2125      +30     
+ Misses        542      522      -20     
- Partials      179      183       +4     

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