pgsql: Insert into getCopyDataMessage() the same logic that already

Log Message:
-----------
Insert into getCopyDataMessage() the same logic that already existed in the
main code path for enlarging libpq's input buffer in one swoop when needing to
read a long data message.  Without this, the code will double the buffer size,
read more data, notice it still hasn't got the whole message, and repeat till
it finally has a large enough buffer.  Which wastes a lot of data-moving
effort and also memory (since malloc probably can't do anything very useful
with the freed-up smaller buffers).  Not sure why this wasn't there already;
certainly the COPY data path is a place where we're quite likely to see long
data messages.  I'm not backpatching though, since this is just a marginal
performance issue rather than a real bug.

Modified Files:
--------------
    pgsql/src/interfaces/libpq:
        fe-protocol3.c (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-protocol3.c?r1=1.33&r2=1.34)