Fix for pg_upgrade with non-writeable cwd

An EntpriseDB testing report indicated that pg_upgrade crashes if it
can't write into the current directory. This only happens in 9.2 and
head, so I have applied the attached patch to fix it.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +