Encapsulate git functions into a Fab-compatible class

@hiker がすでに取り組んでいます。

2026年3月24日 から。

Long term goal is that the SimSys scripts will be replaced with Fab functionality. As a stepping stone towards that, this ticket is about refactoring the existing git functionality in SimSys_Script, creating a Fab Tool implementation, that can seamlessly integrated in Fab later. Using a Fab-like structure means that we only need to maintain one set of functions for the advanced git functions, no need to have one version here and one in Fab.

At the appropriate time, the git class from this repo can just be moved into Fab.

This ticket will **not** create a dependency on Fab, instead one or two required Fab base classes will be copied here. The API for the build system will also remain unchanged, i.e. the change from a set of functions to a class is internal to this repo only.