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>