"tuple concurrently updated" in pg_restore --jobs

I hit this issue intermittently (roughly half the time) while working with a
patch David submitted, and finally found a recipe to reproduce it on an
unpatched v12 instance.

I was surprised to see pg_restore -j2 is restoring ACLs in pre-data in
parallel. Note different session IDs and PIDs:

2020-07-05 23:31:27.448 CDT,"pryzbyj","secondary_dump",24037,"[local]",5f02a91f.5de5,70,,LOG,00000,"statement: REVOKE SELECT ON TABLE pg_catalog.pg_proc FROM PUBLIC; ",,,,,,,,,"pg_restore","client backend"
2020-07-05 23:31:27.448 CDT,"pryzbyj","secondary_dump",24036,"[local]",5f02a91f.5de4,78,,LOG,00000,"statement: GRANT SELECT(tableoid) ON TABLE pg_catalog.pg_proc TO PUBLIC; ",,,,,,,,,"pg_restore","client backend"
2020-07-05 23:31:27.450 CDT,"pryzbyj","secondary_dump",24036,"[local]",5f02a91f.5de4,79,,LOG,00000,"statement: GRANT SELECT(oid) ON TABLE pg_catalog.pg_proc TO PUBLIC; ",,,,,,,,,"pg_restore","client backend"
2020-07-05 23:31:27.450 CDT,"pryzbyj","secondary_dump",24037,"[local]",5f02a91f.5de5,71,,ERROR,XX000,"tuple concurrently updated",,,,,,"REVOKE SELECT ON TABLE pg_catalog.pg_proc FROM PUBLIC;

postgres=# CREATE DATABASE pryzbyj;
postgres=# \c pryzbyj
pryzbyj=# REVOKE ALL ON pg_proc FROM postgres;
pryzbyj=# GRANT SELECT (tableoid, oid, proname) ON pg_proc TO public;
pryzbyj=# \dp+ pg_catalog.pg_proc
Schema | Name | Type | Access privileges | Column privileges | Policies
------------+---------+-------+-------------------+-------------------+----------
pg_catalog | pg_proc | table | =r/postgres | tableoid: +|
| | | | =r/postgres +|
| | | | oid: +|
| | | | =r/postgres +|
| | | | proname: +|
| | | | =r/postgres |

[pryzbyj(at)database ~]$ pg_dump pryzbyj -Fc -f pg_dump.out
[pryzbyj(at)database ~]$ pg_restore pg_dump.out -j2 -d pryzbyj --clean -v
...
pg_restore: entering main parallel loop
pg_restore: launching item 3744 ACL TABLE pg_proc
pg_restore: launching item 3745 ACL COLUMN pg_proc.proname
pg_restore: creating ACL "pg_catalog.TABLE pg_proc"
pg_restore: creating ACL "pg_catalog.COLUMN pg_proc.proname"
pg_restore:pg_restore: while PROCESSING TOC:
finished item 3745 ACL COLUMN pg_proc.proname
pg_restore: from TOC entry 3744; 0 0 ACL TABLE pg_proc postgres
pg_restore: error: could not execute query: ERROR: tuple concurrently updated
Command was: REVOKE ALL ON TABLE pg_catalog.pg_proc FROM postgres;