test: add tests for the majority of the Core assembly

[jonisavo]

The Core assembly is mostly covered by tests now. Some bugs were fixed too.

[codecov-commenter]

Codecov Report

Merging #2 (4fbcc7a) into main (c7d0146) will increase coverage by 24.34%.
The diff coverage is 92.00%.

@@             Coverage Diff             @@
##             main       #2       +/-   ##
===========================================
+ Coverage   22.44%   46.78%   +24.34%     
===========================================
  Files          37       36        -1     
  Lines        1047     1043        -4     
===========================================
+ Hits          235      488      +253     
+ Misses        812      555      -257     

Flag	Coverage Δ
automated	46.78% <92.00%> (+24.34%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...o.savolainen.unitygit/Core/Services/DiffService.cs	100.00% <ø> (+5.88%)	⬆️
...lainen.unitygit/Core/Services/RepositoryService.cs	50.87% <60.00%> (+50.87%)	⬆️
...io.savolainen.unitygit/Core/Internal/GitProcess.cs	90.24% <100.00%> (+0.77%)	⬆️
...lainen.unitygit/Core/Services/GitCommandService.cs	100.00% <100.00%> (+100.00%)	⬆️
...o.savolainen.unitygit/Core/Services/PullService.cs	100.00% <100.00%> (+100.00%)	⬆️
...avolainen.unitygit/Core/Services/RestoreService.cs	100.00% <100.00%> (+100.00%)	⬆️
...savolainen.unitygit/Core/Services/StatusService.cs	100.00% <100.00%> (+100.00%)	⬆️
...ainen.unitygit/Core/Services/UnityGitLogService.cs	100.00% <100.00%> (+95.45%)	⬆️

... and 4 files with indirect coverage changes

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