Ensure git.Repo instances are closed

[cottsay]

This is mostly whitespace changes to use git.Repo instances as a context manager so they're closed properly.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.37500% with 5 lines in your changes missing coverage. Please review.

Project coverage is 38.90%. Comparing base (a3225c3) to head (37586f1).

Files with missing lines	Patch %	Lines
rosdistro_reviewer/yaml_changes.py	0.00%	3 Missing and 1 partial ⚠️
rosdistro_reviewer/git_lines.py	96.42%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   38.90%   38.90%           
=======================================
  Files          12       12           
  Lines         586      586           
  Branches      154      156    +2     
=======================================
  Hits          228      228           
+ Misses        353      352    -1     
- Partials        5        6    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.