Allow opening a bare repo without any worktrees defined.

Right now if you use bare repos with git worktree, lazygit is a bit more annoying to use that it should be.
If I clone down a bare repo and open lazygit in it, it fails with

`Error: must be run inside a git repository`

What would be ideal would be for lazygit to recognize it's a bare repo and go ahead and open.

Then you could use the `w` command to create new worktrees from the branches.

This would lead to a much smoother workflow for those using bare repos with worktrees.