Fix issues with `ga`, `gsp` & `gcf` with git 2.25.1

[sandr01d]

Check list

• [X] I have performed a self-review of my code
• [ ] I have commented my code in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation

Description

We're currently using --pathspec-file-nul in ga, gsp and gcf. This option is not available in older versions of git, such as the current release in the Ubuntu repositories (2.25.1). This PR removes the usage of --pathspec-file-nul in favor of storing the file names passed to the git command in an array instead of a string.
This PR is based on #326. I will rebase and mark it as ready for review, once #326 is merged.

Fixes #328

Type of change

• [X] Bug fix
• [ ] New feature
• [ ] Refactor
• [ ] Breaking change
• [ ] Documentation change

Test environment

• Shell
  • [ ] bash
  • [X] zsh
  • [ ] fish
• OS
  • [X] Linux
  • [ ] Mac OS X
  • [ ] Windows
  • [ ] Others: