Add input prompt for git flow release finish

[twio142]

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 <release-name>
• ...

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!