Function git_config_entries 

pub fn git_config_entries(
    get: impl Fn(&str) -> Option<String>,
    key: &str,
    value: &str,
) -> Vec<(String, String)>

The variables that set key = value on top of the GIT_CONFIG_* entries get already sees. Every slot that names key (case-insensitively, as git does for the section and name) gets the value, since git uses the last one it finds; with none, a slot is appended.