Preview does not work when stashing a deleted file

[sandr01d]

Check list

• [X] I have read through the README
• [X] I have the latest version of forgit
• [X] I have searched through the existing issues

Environment info

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

Problem

When stashing a file that has been deleted, the following error message is displayed instead of a preview:

fatal: ambiguous argument 'README.md': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:                                                          │
'git <command> [<revision>...] -- [<file>...]

Stashing does work as expected though. I will go after this once #326 is merged, so I don't have to do the work twice.

Steps to reproduce

1. Delete any file that is tracked by git
2. Execute gsp and select the file you just deleted