Fixed psql \h SELECT

I added the following test to psql/help.c:

/* if we have an exact match, exit, fixes \h SELECT */
if (strcasecmp(topic, QL_HELP[i].cmd) == 0)
break;

This will exit the help loop if an exact match has already been found.
This prevents \h select from showing SELECT INTO help. Peter, hope you
are OK with this.

-- 
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