We now have a public method for this purpose, so we don't need to
access the private revlog._partialmatch(). Also, I'll probably make
some changes to resolvehexnodeidprefix() later, and I want those to be
reflected by the id() predicate.

Note that this breaks a test case, because we now resolve the prefix
in the unfiltered repo and get an ambiguous lookup, which results in
no revision being added to the revset. The test case was already
documented to be broken even though it wasn't. It's important to note
that {shortest(node)} already uses the unfiltered repo, so we're not
going to break people who get the prefix from there.

I think we may not want to ever use shortest() in the filtered
repo. It seems unlikely to be enough of a win to matter much. For
example, in my hg repo, it would save me only 0.2 hex digits. In
another repo that only I modify, it saves a little more, but it's
still only 0.29 hex digits. It seems unlikely that people will prune
enough commits that only 1/16 of the commits are visible (which is
what it would take a to save a single hex digit). Instead, I'm working
on another approach: allow ambiguous matches to be disambiguated
within a user-specified revset. Whether or not that pans out, I hope
we're okay with this little change in behavior for now and we can
decide what to do about it later.