Add a progress bar for Git actions

### What is the problem or limitation you are having?

Currently, without very verbose logging, actions performed by Git are unaccompanied by a progress bar. For slow internet connections or large repos, this leads users to believe Briefcase is frozen.

### Describe the solution you'd like

Actions performed by Git should have feedback in the console that activity is occurring and the user should not interfere.

### Describe alternatives you've considered

Work in #1855 et al. is limiting the impact of depending on Git.

### Additional context

GitPython is currently used to interact with Git. It supports [adding progress bars](https://gitpython.readthedocs.io/en/stable/reference.html?highlight=progress#git.remote.Remote.push) for arbitrary Git actions.

Start by locating Briefcase's GitPython interactions, especially the arbitrary Git actions that can take time on slow connections or large repositories. Add console feedback for those actions without requiring verbose logging, and verify that users can see activity without interfering with Git operations.

Written by the indexing model from the issue text.