git-gui: Fix the blame window shape.

git-gui: Fix the blame window shape.

On modern high-resolution monitors the blame viewer
window is very high, yet too narrow. This patch
makes it gravitate to a more sane resolution, which
takes the font size into account.

It also changes the default text view size to 80% of
the window, and slightly modifies the border decorations
for better appearance.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>