Function git

pub fn git(source_dir: Option<&Path>) -> BootstrapCommand

Prepares BootstrapCommand that runs git inside the source directory if given.

Whenever a git invocation is needed, this function should be preferred over manually building a git BootstrapCommand. This approach allows us to manage bootstrap-specific needs/hacks from a single source, rather than applying them on next to every git command creation, which is painful to ensure that the required change is applied on each one of them correctly.