narrow: overwrite readfast in excludeddirmanifestctx
The excludeddirmanifestctx does not have underlying storage (because it is
excluded) so the implementation of the `readfast` trying to access it crash.
This was reveled while running some cache warning code on narrow repositories.
That code will be introduced further down in the series.
Differential Revision: https://phab.mercurial-scm.org/D9731
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}