psql: fix variable existence tab completion

psql has the :{?name} syntax for testing for a psql variable existence.  This
commit implements a tab completion for this syntax.  Notably, in order to
implement this we have to remove '{' from WORD_BREAKS.  It appears that
'{' here from the very beginning and it comes from the default value of
rl_basic_word_break_characters.  And :{?name} is the only psql syntax using
the '{' sign.  So, removing it from WORD_BREAKS shouldn't break anything.

Discussion: https://postgr.es/m/CAGRrpzZU48F2oV3d8eDLr%3D4TU9xFH5Jt9ED%2BqU1%2BX91gMH68Sw%40mail.gmail.com
Author: Steve Chavez
Reviewed-by: Erik Wienhold

src/bin/psql/t/010_tab_completion.pl

@@ -413,6 +413,14 @@ check_completion(
 
 clear_query();
 
+# check completion for psql variable test
+check_completion(
+	"\\echo :{?VERB\t",
+	qr/:\{\?VERBOSITY} /,
+	"complete a psql variable test");
+
+clear_query();
+
 # check no-completions code path
 check_completion("blarg \t\t", qr//, "check completion failure path");
 

src/bin/psql/tab-complete.c

@@ -76,7 +76,7 @@
 #endif
 
 /* word break characters */
-#define WORD_BREAKS		"\t\n@><=;|&{() "
+#define WORD_BREAKS		"\t\n@><=;|&() "
 
 /*
  * Since readline doesn't let us pass any state through to the tab completion
@@ -1786,6 +1786,8 @@ psql_completion(const char *text, int start, int end)
 			matches = complete_from_variables(text, ":'", "'", true);
 		else if (text[1] == '"')
 			matches = complete_from_variables(text, ":\"", "\"", true);
+		else if (text[1] == '{' && text[2] == '?')
+			matches = complete_from_variables(text, ":{?", "}", true);
 		else
 			matches = complete_from_variables(text, ":", "", true);
 	}