pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e70abd67c3e6155fe8e853c4e29255578d9cf48d

Modified Files
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src/test/isolation/isolation_main.c | 37 +++++++++++++--------------------
src/test/regress/pg_regress_main.c  | 41 +++++++++++++++----------------------
2 files changed, 30 insertions(+), 48 deletions(-)