Function git2_hooks::hooks_commit_msg

pub fn hooks_commit_msg(
    repo: &Repository,
    other_paths: Option<&[&str]>,
    msg: &mut String
) -> Result<HookResult, HooksError>

this hook is documented here https://git-scm.com/docs/githooks#_commit_msg we use the same convention as other git clients to create a temp file containing the commit message at <.git|hooksPath>/COMMIT_EDITMSG and pass it’s relative path as the only parameter to the hook script.