git-gui: Never line wrap in file lists.

git-gui: Never line wrap in file lists.

Some of my file paths in some of my repositories are very long, this
is rather typical in Java projects where the path name contains a deep
package structure and then the file name itself is rather long and
(hopefully) descriptive.  Seeing these paths line wrap in the file lists
looks absolutely horrible.  The entire rendering is almost unreadable.

Now we draw both horizontal and vertical scrollbars for both file lists,
and we never line wrap within the list text itself.

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