Preview for `git add` is broken for files that contain backslashes in their name

[sandr01d]

When trying to add a file that has a backslash in its name, the preview will not be shown correctly. Instead the preview shows something like:

error: Could not access 'file\\.txt'

Steps to reproduce

1. Create a file with a backslash in its name
   echo test > file\\.txt
2. Try to stage the file using git forgit add
3. Observe that the preview shows an error message instead of the files content.

[carlfriedrich]

This can be approached the same way like I suggested in #465:

1. Refactor the code to have testable units
2. Write a failing unit and/or integration test
3. Fix the code
4. See the test pass

The key work here is (1), as _forgit_add has a rather large command chain to generate the fzf input.

[rikas072170]

Melbet code?