git-gui: Cleanup diff construction code to prepare for more options.

git-gui: Cleanup diff construction code to prepare for more options.

I'd like to allow the user to have more control over how we format
the diff in the diff viewer; to that end we need to add additional
options to the diff-index command line as we construct the command
for execution.

So cleanup the command handling code now to use lappend so we can
come back and add in our additional options.

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