Function vimdiff_resolve_layout 

pub fn vimdiff_resolve_layout<'a>(
    tool: &str,
    mergetool_layout: Option<&'a str>,
    vimdiff_layout_fallback: Option<&'a str>,
) -> &'a str

Resolves the layout string for a merge tool name, matching merge_cmd in Git’s vimdiff script.

• tool — e.g. vimdiff, gvimdiff2, nvimdiff1.
• mergetool_layout — value of mergetool.<tool>.layout when set.
• vimdiff_layout_fallback — value of mergetool.vimdiff.layout when variant-specific layout is unset.