Preserve attstattarget on REINDEX CONCURRENTLY

Hello !

We encountered the following bug recently in production: when running REINDEX
CONCURRENTLY on an index, the attstattarget is reset to 0.

Consider the following example:

junk=# \d+ t1_date_trunc_idx
Index "public.t1_date_trunc_idx"
Column | Type | Key? | Definition
| Storage | Stats target
------------+-----------------------------+------
+-----------------------------+---------+--------------
date_trunc | timestamp without time zone | yes | date_trunc('day'::text, ts)
| plain | 1000
btree, for table "public.t1"

junk=# REINDEX INDEX t1_date_trunc_idx;
REINDEX
junk=# \d+ t1_date_trunc_idx
Index "public.t1_date_trunc_idx"
Column | Type | Key? | Definition
| Storage | Stats target
------------+-----------------------------+------
+-----------------------------+---------+--------------
date_trunc | timestamp without time zone | yes | date_trunc('day'::text, ts)
| plain | 1000
btree, for table "public.t1"

junk=# REINDEX INDEX CONCURRENTLY t1_date_trunc_idx;
REINDEX
junk=# \d+ t1_date_trunc_idx
Index "public.t1_date_trunc_idx"
Column | Type | Key? | Definition
| Storage | Stats target
------------+-----------------------------+------
+-----------------------------+---------+--------------
date_trunc | timestamp without time zone | yes | date_trunc('day'::text, ts)
| plain |
btree, for table "public.t1"

I'm attaching a patch possibly solving the problem, but maybe the proposed
changes will be too intrusive ?

Regards,

--
Ronan Dunklau