Simplify code building the LR conflict messages

While reviewing another conflict-related thread, I noticed that the
existing conflict messages are currently getting built using
unexpected logic.

e.g
----------
if (tuple_value.len > 0)
{
appendStringInfoString(&tuple_value, "; ");
appendStringInfo(&tuple_value, _("existing local row %s"),
desc);
}
else
{
appendStringInfo(&tuple_value, _("Existing local row %s"),
desc);
}
----------

I couldn't think of a reason why the "; " string needed to be
separated from the rest of the message like that. And when you combine
the strings, the logic easily collapses into a single statement with
less code and greater readability.

There were several code fragments like this. Here is a patch to modify them.

Thoughts?

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Kind Regards,
Peter Smith.
Fujitsu Australia