Suggested fix for \p and \r in psql

Hi,

I've noticed two issues with the query buffer post-commit e984ef5
(Support \if ... \elif ... \else ... \endif in psql scripting):

1. \p ignores the "previous buffer". Example:

postgres=# select 1;
?column?
----------
1
(1 row)

postgres=# \p
Query buffer is empty.

That doesn't match the pre-commit behavior, and is not
consistent with \e or \w

2. \r keeps the "previous buffer". I think it should clear it. Example:

postgres=# select 1;
?column?
----------
1
(1 row)

postgres=# select 2 \r
Query buffer reset (cleared).
postgres=# \w /tmp/buffer
postgres=# \! cat /tmp/buffer
select 1;

I suggest the attached fix, with a few new regression tests.

Best regards,
--
Daniel Vérité
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