git-gui: Refactor diff pane popup support for future improvements

git-gui: Refactor diff pane popup support for future improvements

The current popup_diff_menu procedure is somewhat messy as it has a
few duplications of the same logic in each of the different legs of
the routine.  We can simplify these by setting a few state variables
in the different legs.

No functional change, just a cleanup to make it easier to implement
future functional changes within this block.

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