.arcconfig

subrepo: drop the default value of None for the archive matcher

This was flagged by pytype after adding hints to `match.subdirmatcher` that it
takes a non-optional matcher.  That matcher argument is used without a guard in
the subdirmatcher constructor, so that's the correct restriction.

I don't think this fixes a bug in practice because the only way these are
invoked is either by a parent `hgsubrepo.archive()`, `archival.archive()`, or
the largefiles override of these.  The `hgsubrepo.archive()` case (and the
largefiles override) uses what the caller provided, so the caller will
eventually be `archival.archive()` (or the largfiles override) up the call
chain.  The `archival.archive()` method also has None for its matcher's default
arg.  However, the three callers of that (`commands.archive()`,
`webcommands.archive()`, and `extdiff.snapshot()`) all provide a matcher
argument, so the None case can never occur unless a 3rd party extension swaps it
for None.  Sadly, we can't make the argument on the `archival.archive()`
non-optional because there is a kwarg prior to it.

Even though the largefiles override of `archival.archive()` is provided a valid
matcher, we duplicate the internal creation of the matcher that the original
`archival.archive()` does for consistency.  By eliminating an impossible to hit
case, we can simplify some of the subrepo code too, by dropping unreachable
code.

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    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}