Add input prompt for `git flow release finish`

**Is your feature request related to a problem? Please describe.**

When running lazygit in a tmux popup window, any command that spawns a nvim instance doesn't work.

It seems like nvim is running and even accept keys, but the interface won't load properly and only shows a blank window.


The same commands work just fine in a normal tmux pane.

The following commands are causing such behavior:

- `git commit --amend`
- `git flow release finish `
- ...

Steps to reproduce the behavior:

1. Launch lazygit in a tmux popup window: `tmux popup lazygit`.
2. On a git-flow release branch, opt to finish the release.

**Describe the solution you'd like**

I understand that tmux popup windows often act unexpectedly and are tricky to deal with. I can surely amend my commit message with the built-in input prompt. So it'd be nice if `git flow release finish` also invokes an input prompt for the tag message.

Thank you!