[PATCH 0 of 2] Add optional choices parameter to ui.prompt()

[Steve Borho]

The first patch makes it possible for applications which subclass ui to
build multiple choice dialogs in the circumstances where it is
warranted.  merge.py is one such candidate, so I included a patch
which adds choices to that module's ui.prompt() calls.

I'm not 100% pleased with the result, as the choices are somewhat
redundant with the pat argument and message text, but this doesn't
seem like a good enough excuse to break backwards compatibility.

I'm also not 100% sure the second patch is correct.  Please don't
push it.  It's mostly included to generate comments.

--
Steve Borho