See the following in 2. Changes after 1.8:

Various functions have been moved from cmdutil.py to scmutil.py, including revrange/revsingle/revpair and match/matchall/matchfiles

Also - at the top of the page, it seems we don't have a process for detecting breaking changes in mercurial:

How to keep your extension compatible

• Subscribe to this page to keep up-to-date on changes
• Mark your calendar to test your extension before our scheduled releases (Mar 1, Jul 1, Nov 1)
• Use introspection in helper functions to choose which interface to call, for example:

def updatedir(*args, **kwargs):
    if hasattr(patch, 'updatedir'):
        patch.updatedir(*args, **kwargs)
    else:
        cmdutil.updatedir(*args, **kwargs)

• Use inspect.getargspec() to deal with changing argument lists