Renaming a table to an array's autogenerated name

In commit 9aa3c782c93, Tom fixed a bug in which creating a table _foo
when an array type of that name already existed would make the array
type change its name to get out of the way. But it missed a trick in
that renaming a table would still cause a conflict.

Steps to reproduce:

postgres=# create table foo (id int);
CREATE TABLE
postgres=# create table bar (id int);
CREATE TABLE
postgres=# alter table bar rename to _foo;
ERROR: type "_foo" already exists

Attached is a patch that fixes this bug.

One interesting thing to note however, is that if you rename a table to
its own array's name (which was my case when I found this bug), the new
array name will be ___foo instead of just __foo. I don't know if it's
worth worrying about that.
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Vik Fearing +33 6 46 75 15 36
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