Feature: GitDevClean command

[b0o]

It would be nice to have a GitDevClean command to clean the current repo if no args are passed, which would close all buffers that are open containing files from the repo and then delete the repo directory.

The command could also support an argument to clean a specific repo, e.g. GitDevClean moyiz/git-dev.nvim would only clean the given repo.

Additionally, it would be nice if the repo name was tab-completable from the list of local cached repos.