Switch from binary git to gitoxide

[rimutaka]

Relying on a git app being installed works fine if stm runs as a full trust application, but being partial trust (windows store, snap, see #11 for more) requires GIT to be included in the package. That's doable, but is a hassle. We only need so many basic git features gitoxide should be able to provide.

https://github.com/Byron/gitoxide

1. Check if everything we need is there
2. Try using some of their modules
3. Switch over

Relates to #11