Consider using different git implementations

**Is your feature request related to a problem? Please describe.**
Weblate currently executes git internally for most of the operations. There might be better ways to interact with the repo these days using native libraries that would avoid forking a new process for every operation.

**Describe the solution you'd like**

Evaluate possible options, right now I can see:

* https://www.pygit2.org/, based on libgit2, seems promising
* https://github.com/gitpython-developers/GitPython, in maintenance mode, memory leaks, probably not a good choice
* https://github.com/dulwich/dulwich, no new release for two years, but there is some development

**Additional context**

The only reason why Weblate didn't use some library from the beginning is that back in time when Weblate was initially written, there was no suitable solution.