pgsql: In psql tab completion, offer spelled-out commands not abbreviat

In psql tab completion, offer spelled-out commands not abbreviations.

Various psql backslash commands have both single-letter and long
forms, for example \e and \edit. Previously, tab completion
generally offered the single-letter form but not the long form.
It seems more sensible to offer the long form, because (a) no
useful completion can happen when you've already typed the single
letter, and (b) if you're not so familiar with the command set
as to know that, the long form is likely to be less confusing.

Haiying Tang, reviewed by Dagfinn Ilmari Mannsåker and myself

Discussion: https://postgr.es/m/OS0PR01MB61136018064660F095CB57A8FB129@OS0PR01MB6113.jpnprd01.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7cffa2ed0c9f7f4d96bac7af5284c47e82af5ffa

Modified Files
--------------
src/bin/psql/tab-complete.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)