git-gui: Move on to the next filename after staging/unstaging a change

git-gui: Move on to the next filename after staging/unstaging a change

Suppose the "Unstaged Changes" pane contains a list of files, and one of
them is selected (i.e., that diff is currently being displayed). If one
clicks on the icon to stage the change, git-gui clears the diff and one
has to click on another filename to see the next diff in the list.

This patch changes that behaviour. If one clicks on the icon to stage
(or unstage) the file whose diff is being displayed, git-gui will move
on to the next filename in the list and display that diff instead of a
blank diff pane. If the selected file was at the end of the list, the
diff pane will display the previous diff instead; if the selected file
was the only one listed, the diff pane will become blank.

If no diff is currently being displayed, this patch changes nothing.

Signed-off-by: Abhijit Menon-Sen <ams@toroid.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>