BUG #6624: Tab completion of identifier containing single backslash triggers warnings

The following bug has been logged on the website:

Bug reference:      6624
Logged by:          Stuart Bishop
Email address:      stuart@stuartbishop.net
PostgreSQL version: 9.1.3
Operating system:   Ununtu
Description:=20=20=20=20=20=20=20=20

psql tab completion can emit WARNING messages if escape_string_warning is
set to true. The trigger seems to be an identifier containing a single
backslash.

# CREATE SEQUENCE "\foo";
CREATE SEQUENCE
# \ds "\

(at this point, hit <TAB> to trigger completion)

# drop sequence "\WARNING:  nonstandard use of \\ in a string literal
LINE 1: ... substring(pg_catalog.quote_ident(c.relname),1,2)=3D'"\\' AND ...
                                                             ^
HINT:  Use the escape string syntax for backslashes, e.g., E'\\'.
WARNING:  nonstandard use of \\ in a string literal
LINE 3: ...ing(pg_catalog.quote_ident(n.nspname) || '.',1,2)=3D'"\\' AND ...
                                                             ^
HINT:  Use the escape string syntax for backslashes, e.g., E'\\'.
WARNING:  nonstandard use of \\ in a string literal
LINE 3: ...alog.quote_ident(nspname) || '.',1,2) =3D substring('"\\',1,pg...
                                                             ^
HINT:  Use the escape string syntax for backslashes, e.g., E'\\'.
WARNING:  nonstandard use of \\ in a string literal
LINE 5: ... || '.' || pg_catalog.quote_ident(c.relname),1,2)=3D'"\\' AND ...
                                                             ^
HINT:  Use the escape string syntax for backslashes, e.g., E'\\'.
WARNING:  nonstandard use of \\ in a string literal
LINE 5: ...og.quote_ident(n.nspname) || '.',1,2) =3D substring('"\\',1,pg...
                                                             ^
HINT:  Use the escape string syntax for backslashes, e.g., E'\\'.
WARNING:  nonstandard use of \\ in a string literal
LINE 5: ...alog.quote_ident(nspname) || '.',1,2) =3D substring('"\\',1,pg...
                                                             ^
HINT:  Use the escape string syntax for backslashes, e.g., E'\\'.
foo" ;