git-gui: Refactor current_diff -> current_diff_path.

git-gui: Refactor current_diff -> current_diff_path.

We now need to keep track of which side the current diff is for,
HEAD<->index or index<->working directory.  Consequently we need
an additional "current diff" variable to tell us which side the
diff is for.  Since this is really only necessary in reshow_diff
I'm going to declare a new global, rather than try to shove both
the path and the side into current_diff.

To keep things clear later on, I'm renaming current_diff to
current_diff_path.  There is no functionality change in this
commit.

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