Fix git helpers bugs

[hampustagerud]

Hello, I found some issues with git using the file header rule 🙂

• Info not being fetched when using stdin (noticed when using the cli to lint in git hooks)
• Defaults not working when file is not committed (it should default to the current git user)
  • Also added a test for this

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.03704% with 7 lines in your changes missing coverage. Please review.

Project coverage is 94.78%. Comparing base (5ed94a0) to head (fb6f199). Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
Sources/GitFileInfo.swift	75.86%	7 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1883      +/-   ##
===========================================
- Coverage    95.14%   94.78%   -0.37%     
===========================================
  Files          134      134              
  Lines        18985    19016      +31     
===========================================
- Hits         18063    18024      -39     
- Misses         922      992      +70     

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[nicklockwood]

Amazing - thank you!