I was wrong; I thought I figured out the Git script I used the most everyday. Turns out, it is this:

w

It has a variation:

w 'Some commit message'

Running w alone uses WIP as the default commit message.

I wrote it as a fish shell function, but it should be easy to convert to a shell script:

#w and ww are the same. Except the latter has a '-n' option
function w
    #set --local staged (gs | cut -c1 | ag -v "\?" | string collect | string trim)
    set --local staged (git status -s | grep "^[MADRCU]" | string collect | string trim)
    if test -n "$staged"
        #echo "something staged"
    else
        #echo "nothing staged"
        git add .
    end

    if not string length -q -- "$argv"
        gc -m 'WIP' 
    else
        gc -m "$argv"
    end
end