Add relative_to_project_root configs

[shunichironomura]

• Fix search_for_project_root
• add diff_file to output
• add relative_to_project_root config

[codecov]

Codecov Report

Attention: Patch coverage is 70.27027% with 11 lines in your changes missing coverage. Please review.

Project coverage is 78.68%. Comparing base (a5340d4) to head (e569597).

Files	Patch %	Lines
capsula/_context/_git.py	57.14%	3 Missing ⚠️
capsula/_run.py	66.66%	3 Missing ⚠️
capsula/_cli.py	0.00%	2 Missing ⚠️
capsula/_context/_command.py	84.61%	2 Missing ⚠️
capsula/_context/_file.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
+ Coverage   78.51%   78.68%   +0.16%     
==========================================
  Files          29       29              
  Lines         945      971      +26     
==========================================
+ Hits          742      764      +22     
- Misses        203      207       +4     

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