narrow: overwrite readfast in excludeddirmanifestctx...
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