[PATCH 0 of 5 V3] ui: introduce "extractchoices()" for "promptchoice()" users

[FUJIWARA Katsunori]

This patch series introduces "extractchoices()" for "promptchoice()"
users wanting list of choices to display help.

Difference between V2 and V3:

  - define "ui.extractchoices()" as static method for reusability

    for example, reuse from the tool checking translation of the
    string passed to "ui.promptchoice()". please see the patch series
    below, too:

      http://selenic.com/pipermail/mercurial-devel/2013-November/055103.html

Difference between V1 and V2:

  - use "strip" extension instead of "mq" to use "hg strip"