psql: include intra-query "--" comments in what's sent to the server.

psql's lexer has historically deleted dash-dash (single-line) comments
from what's collected and sent to the server.  This is inconsistent
with what it does for slash-star comments, and people have complained
before that they wish such comments would be captured in the server log.
Undoing the decision completely seems like too big a behavioral change,
however.  In particular, comments on lines preceding the start of a
query are generally not thought of as being part of that query.

What we can do to improve the situation is to capture comments that
are clearly *within* a query, that is after the first non-whitespace,
non-comment token but before the query's ending semicolon or backslash
command.  This is a nearly trivial code change, and it affects only a
few regression test results.

(It is tempting to try to apply the same rule to slash-star comments.
But it's hard to see how to do that without getting strange history
behavior for comments that cross lines, especially if the user then
starts a new query on the same line as the star-slash.  In view of
the lack of complaints, let's leave that case alone.)

Discussion: https://postgr.es/m/CAJcOf-cAdMVr7azeYR7nWKsNp7qhORzc84rV6d7m7knG5Hrtsw@mail.gmail.com

contrib/pg_stat_statements/expected/pg_stat_statements.out

@@ -101,7 +101,7 @@ SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
 ------------------------------------------------------------------------------+-------+------
  PREPARE pgss_test (int) AS SELECT $1, $2 LIMIT $3                            |     1 |    1
  SELECT $1                                                                   +|     4 |    4
-                                                                             +|       | 
+   -- multiline                                                              +|       | 
    AS "text"                                                                  |       | 
  SELECT $1 + $2                                                               |     2 |    2
  SELECT $1 + $2 + $3 AS "add"                                                 |     3 |    3