Remove JGit and use only the Git CLI

[mnonnenmacher]

JGit was introduced as it provides a nicer way to interact with Git than running Git CLI commands as external processes. Initially, the plan was that it would at some point completely replace the usage of the Git CLI, but this is prevented by missing support for features like sparse checkout.

The simultaneous use of both Git implementations increases the complexity of the Git plugin and makes it hard to debug issues as both tools need to be understood. This is especially a problem when debugging authentication issues, as both tools use different ways to access credentials.

Therefore, the proposal is to stop using JGit and solely rely on the Git CLI, because it supports all features needed by ORT and because using only one tool would reduce the complexity of the Git implementation.

[heliocastro]

I'm all for having a single way to handle things, and configuration-wise, it would facilitate too, as we can reproduce git issues without needing to be in Ort space.

[sschuberth]

Not a strict requirement, but when creating a new / refactored Git implementation around the git CLI, it could also make sense to look into #4668 again in that context.