subrepo: add shortid() method to subrepo classes
This method takes an "id" (e.g. a revision id) and returns a "short" version
(e.g. a short revision id).
This will be used on the next revision to fix a small bug in the way that the
text on the promptchoice shown when a subrepo diverges is generated.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html