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.
all()
draft()
::tip
draft() and ::tip
::tip and draft()
0::tip
roots(0::tip)
author(lmoscovicz)
author(mpm)
author(lmoscovicz) or author(mpm)
author(mpm) or author(lmoscovicz)
tip:0
max(tip:0)
min(0:tip)
0::
min(0::)
roots((tip~100::) - (tip~100::tip))
::p1(p1(tip))::
public()
:10000 and public()
draft()
:10000 and draft()