[BUG] git gui: "unstage line" not working on newly-added file

When looking using git-gui at a file that does not exist in HEAD and
has been added to the index, lines cannot be selectively staged out of
the index.

It would look like outside of the gui such (or similar) behaviour
would be accessible from an interactive mode in reset, but it looks
like we do not have that.

I was able, however, to "add -N" the file and selectively stage the
lines that I wanted using git-gui.