Describe the bug
Trying to launch gitui inside a sparse checkout produces the following message:

invalid path
please run gitui inside of a non-bare git repository

To Reproduce
Steps to reproduce the behavior:

$ git clone \
	--depth 1 \
	--filter=blob:none \
	--sparse \
	--branch master \
	https://github.com/extrawurst/gitui.git \
	~/gitui
$ cd ~/gitui
$ git sparse-checkout set --stdin <<EOF
.gitignore
EOF
$ gitui
invalid path
please run gitui inside of a non-bare git repository
$ lazygit
...just works...

Expected behavior
gitui should detect the repo and launch without error.

Screenshots
None

Context (please complete the following information):

• Ubuntu 20.04
• gitui 0.20.1
• Rust version: Not installed

Additional context
Installed via nix.