dump cannot be restored if schema permissions revoked

Hello hackers,

I noticed that in some situations involving the use of REVOKE ON SCHEMA,
pg_dump
can produce a dump that cannot be restored. This prevents successful
pg_restore (and by corollary, pg_upgrade).

An example shell script to recreate this problem is attached. The error
output appears at the end like this:

<snippet>
+ pg_restore -d postgres /tmp/foo.dmp
pg_restore: [archiver (db)] Error while PROCESSING TOC:
pg_restore: [archiver (db)] Error from TOC entry 2748; 0 0 ACL TABLE
mytable owneruser
pg_restore: [archiver (db)] could not execute query: ERROR: permission
denied for schema private
Command was: GRANT SELECT ON TABLE private.mytable TO privileged WITH
GRANT OPTION;
SET SESSION AUTHORIZATION privileged;
GRANT SELECT ON TABLE private.mytable TO enduser WITH GRANT OPTION;
RESET SESSION AUTHORIZATION;
WARNING: errors ignored on restore: 1
-bash-4.2$
</snippet>

Note that `privileged` user needs to grant permissions to `enduser`, but
can no longer do so because `privileged` no longer has access to the
`private` schema (it was revoked).

How might we fix up pg_dump to handle these sorts of situations? It seems
like pg_dump might need extra logic to GRANT the schema permissions to the
`privileged` user and then REVOKE them later on?

Thanks for looking,
--Richard