pgsql: Fix behavior of psql's \p to agree with \g, \w, etc.

Fix behavior of psql's \p to agree with \g, \w, etc.

In commit e984ef586 I (tgl) simplified the behavior of \p to just print
the current query buffer; but Daniel Vérité points out that this made it
inconsistent with the behavior of \g and \w. It should print the same
thing \g would execute. Fix that, and improve related comments.

Daniel Vérité

Discussion: https://postgr.es/m/9b4ea968-753f-4b5f-b46c-d7d3bf7c8f90@manitou-mail.org

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/5dbc5da1187c1ddb6e091047194d364337ebf232

Modified Files
--------------
src/bin/psql/command.c | 25 +++++++++++++++++++------
src/test/regress/expected/psql.out | 32 ++++++++++++++++++++++++++++++++
src/test/regress/sql/psql.sql | 12 ++++++++++++
3 files changed, 63 insertions(+), 6 deletions(-)