git-gui: Brown paper bag fix division by 0 in blame

git-gui: Brown paper bag fix division by 0 in blame

If we generate a blame status string before we have obtained
any annotation data at all from the input file, or if the input
file is empty, our total_lines will be 0.  This causes a division
by 0 error when we blindly divide by the 0 to compute the total
percentage of lines loaded.  Instead we should report 0% done.

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