git-gui: Start blame windows as tall as possible

git-gui: Start blame windows as tall as possible

Most users these days are using a windowing system attached to a
monitor that has more than 600 pixels worth of vertical space
available for application use.  As most files stored by Git are
longer than they are wide (have more lines than columns) we want
to dedicate as much vertical space as we can to the viewer.

Instead of always starting the window at ~600 pixels high we now
start the window 100 pixels shorter than the screen claims it has
available to it.  This -100 rule is used because some popular OSen
add menu bars at the top of the monitor, and docks on the bottom
(e.g. Mac OS X, CDE, KDE).  We want to avoid making our window too
big and causing the window's resize control from being out of reach
of the user.

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