The expandnarrow functionality lets a client have a shorthand (for
Google it's a reference to a checked-in file) for a set of includes
and excludes. For testing we should probably implement a simple
version of that functionality here. For now, rename the capability so
we don't burn the good name in the future if we need to change
behavior.
It's plausible that this functionality should be dropped from the
narrowhg we ship long-term, but I'm dubious as it seems pretty likely
other organizations will want similar shorthands for commonly-used
subsets of their trees.