git-gui: Allow the user to change the diff viewer font size.

git-gui: Allow the user to change the diff viewer font size.

Because the diff area is one of the most important areas to be able to
read users should be able to increase or decrease the size of the font
used within that area.

Currently we save that back to the global configuration, even if it
may have originated from the local repository configuration.  This
is probably going to be considered to be a bug by at least one user
who wants some sort of different font within a given repository, but
I'm just going to ignore the problem for now.

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