Use extensible buffers to assemble command lines

Use extensible buffers to assemble command lines

This makes use of StringInfo to assemble command lines, instead of
using fixed-size buffers and the (remote) possibility of "command too
long" errors.  Also makes the code a bit simpler.

This covers the test driver programs pg_regress and
pg_isolation_regress.

Similar to the changes done for pg_rewind in a33e17f210.

Discussion: https://www.postgresql.org/message-id/2be4fee5-738f-4749-b9f8-b452032c7ade%40eisentraut.org