Assume github for git checkout when no URL is given

[gausie]

Add tests for that and more

[codecov]

Codecov Report

Merging #1014 (6846907) into main (25f3ed6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1014   +/-   ##
=========================================
  Coverage     28.50%   28.50%           
- Complexity    13565    13570    +5     
=========================================
  Files          1031     1031           
  Lines        160600   160602    +2     
  Branches      35276    35278    +2     
=========================================
+ Hits          45773    45784   +11     
+ Misses       106385   106379    -6     
+ Partials       8442     8439    -3     

Impacted Files	Coverage Δ
...ourceforge/kolmafia/textui/command/GitCommand.java	70.58% <100.00%> (+19.56%)	⬆️

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