Consider using different git implementations

[nijel]

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.

[comradekingu]

https://github.com/git/git/blob/master/Documentation/technical/protocol-v2.txt ?