fix: resolve --file from repository subdirectories

[ychampion]

This Pull Request fixes/closes #2871.

It changes the following:

• resolves --file paths against the repository root when GitUI starts from a subdirectory
• keeps current-directory-relative --file paths working
• adds a regression test for both path forms

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

Validation:

• make check
• git diff --check