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