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

src/fe_utils/psqlscan.l

@@ -378,12 +378,11 @@ other			.
 					/*
 					 * Note that the whitespace rule includes both true
 					 * whitespace and single-line ("--" style) comments.
-					 * We suppress whitespace at the start of the query
-					 * buffer.  We also suppress all single-line comments,
-					 * which is pretty dubious but is the historical
-					 * behavior.
+					 * We suppress whitespace until we have collected some
+					 * non-whitespace data.  (This interacts with some
+					 * decisions in MainLoop(); see there for details.)
 					 */
-					if (!(output_buf->len == 0 || yytext[0] == '-'))
+					if (output_buf->len > 0)
 						ECHO;
 				}