provide an easy way to check if MR already exists for the current branch

[sding3]

There doesn't appear to be an easy to check if there already an MR associated with the current branch.

This is currently how I'm doing it, which is a bit cumbersome.

function labmrexists() {(
    set +e

    output="$(lab mr show --no-pager 2>&1)"
    if [[ $? -eq 0 ]]; then
        echo yes # exists
        return 0
    fi
    if [[ $output == *"cannot determine MR id"* ]]; then
        echo no # does not exist
        return 0
    fi
    echo $output # not sure
    return 1
)}

It would be helpful if lab mr show can use different exit codes when there is no MR v.s. when it failed to perform the check due to some errors.

Additionally, it would be great if there is a command for me to get at the MR URL, e.g. lab mr show --format '{{.mr.url}}' or something like that.