git-gui: Disable diff actions when no diff is active.

git-gui: Disable diff actions when no diff is active.

There is no reason why the user should be able to operate on the diff
buffer if there is no currently selected diff; likewise the "File:"
label text appears rather silly looking all by itself when no diff
is being shown in the diff buffer.

So now we only enable widgets (like menu items) if there is a diff
currently showing, and we disable them when a diff isn't showing.

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