Refactor git config

[queil]

• - make the rooz system-config gitconfig into a separate volume so it can be just mounted to work containers as-is
• - move the rooz internally used sshCommand separate from the gitconfig so the config can be used both by rooz and the user (to allow user-specific settings like aliases, user name/email configuration)
• - make mounting .gitconfig opt-out via system config (in case the user wants to manage the config themselves)

[queil]

Won't fix