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.
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.