git-gui: Use a grid layout for the blame viewer.

git-gui: Use a grid layout for the blame viewer.

Using a panedwindow to display the blame viewer's individual columns
just doesn't make sense.  Most of the important data fits within the
columns we have allocated, and those that don't the leading part fits
and that's good enough.  There are just too many columns within this
viewer to let the user sanely control individual column widths.  This
change shouldn't really be an issue for most git-gui users as their
displays should be large enough to accept this massive dump of data.

We now also have a properly working horizontal scrollbar for the
current file data area.  This makes it easier to get away with a
narrow window when screen space is limited, as you can still scroll
around within the file content.

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