git-gui: Use vi-like keys in merge dialog

git-gui: Use vi-like keys in merge dialog

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 <spearce@spearce.org>