Untracked files in repos break plugin updates

## Problem

If a plugin has untracked or otherwise changed files in the local repository, it cannot be updated.

For example:

```
- Finishing ... Done!
x lsp_signature.nvim:
error: The following untracked working tree files would be overwritten by merge:
doc/tags
Please move or remove them before you merge.
Aborting
Updating a793d02..62cadce
```

## Proposal

Run `git clean` and `git reset --hard` commands under the hood to remove any unexpected local changes when performing updates.