Extend test coverage of OpenQA::Git

[r-richardson]

Related Ticket: https://progress.opensuse.org/issues/164886

[okurz]

This was created as alternative approach to

• WIP Extend test coverage of openQA::Git #5851

I recommend against moving more tests into that file as long as they do not rely on a test database

[perlpunk]

Only one style suggestion for next time

[perlpunk]

I guess we have to force merge this because of the broken circleci test

Edit: @r-richardson fixed it

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (9a04e5e) to head (f5a6444).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5854   +/-   ##
=======================================
  Coverage   98.50%   98.51%           
=======================================
  Files         395      395           
  Lines       38767    38782   +15     
=======================================
+ Hits        38188    38205   +17     
+ Misses        579      577    -2     

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