The check for the number of roles in the target cluster for an upgrade
selects the existing roles and performs a COUNT(*) over the result. A
value of one is the expected query result value indicating that only
the install user is present in the new cluster. The result was converted
with the function for converting a string containing an Oid into a numeric,
which avoids potential overflow but makes the code less readable since
it's not actually an Oid at all.
Discussion: https://postgr.es/m/41AB5F1F-4389-4B25-9668-5C430375836C@yesql.se
106f26a849