git-gui: Paper bag fix info dialog when no files are staged at commit

git-gui: Paper bag fix info dialog when no files are staged at commit

If the user tries to commit their changes without actually staging
anything we used to display an informational dialog suggesting they
first stage those changes, then retry the commit feature.

Unfortunately I broke this in aba15f7 ("Ensure error dialogs always
appear over all other windows") and failed to fix it in the paper
bag fix that came one day after it.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>