Don't rely on system git executable

[doshitan]

Context

The project currently has a bunch of utilities for interacting with Git in nava/platform/util/git.py. These call the git executable in a subprocess. This requires users have a particular version of git available on their system to use the tool. We can relax that requirement, and possibly improve some performance, by switching to library for interactions with Git.

Although ultimately copier still requires the git executable at the moment, this tool may eventually have useful functionality that interacts with git outside of Copier. In addition to not having to re-invent a Python interface to Git operations.

Implementation

Probably with dulwich (https://github.com/jelmer/dulwich)

Possibly with pytgit2 (https://www.pygit2.org/index.html)

Copier has a similar open issue, though there's not much interest in making the switch there because (sensibly) if someone is using a Git template, they almost certainly have git installed.