This patch changes gp_dump_query_oids by traversing the parsed query
tree instead of traversing the query tree struct, which has too
many node type and corner cases to consider. Even it is a little bit
risky to traverse the pased query tree string, we haven't seen any
sign that postgresql upstream is going to change the format.

In addition, we also fix a minirepro python script bug when column
stats has text type most common values that containing single quote,
minirepro failed to escape that text, which causes sql grammar error
and can't insert that statistics.

Update minirepro to deal with error messages correctly, set PGUSER
as default user, let output file accept relative path.
Also updated minirepro behave test, make it pass.

Closes #1024