Function add 

pub fn add(
    layout: &Layout,
    repo: &str,
    name: &str,
    url: &str,
) -> Result<bool, String>

Adds one remote to one repository, and the marker if it is not there.

Returns whether the marker was written now: the daemon reads it at start, so a marker written now means a restart before landings follow.

Errors

Returns a description when the repository is not under the root, the name is already a remote, or git refuses.