The imerge extension doesn't work right on Solaris because it
relies on having a tool called "merge" in the user's path, and
that's from GNU RCS and isn't a default part of a Solaris install.

The code in imerge.py (credit to Rich Lowe for finding this) is
this:

        # this could be greatly improved
        realmerge = os.environ.get('HGMERGE')
        if not interactive:
            os.environ['HGMERGE'] = 'merge'

It just overrides the $HGMERGE variable with 'merge'.  That won't
work on all systems.  Something better is needed here.  (Or Mercurial
install needs to depend on RCS installation ...)