Since we support vi-like keys for scrolling in other UI contexts we can easily do so here too. Tk's handy little `event generate' makes this a lot easier than I thought it would be. We may want to go back and fix some of the other vi-like bindings to redirect to the arrow and pageup/pagedown keys, rather than running the view changes directly.
I've bound 'v' to visualize, as this is a somewhat common thing to want to do in the merge dialog. Control (or Command) Return is also bound to start the merge, much as it is bound in the main window to activate the commit.
Signed-off-by: Shawn O. Pearce <email@example.com>