pg_regress: Don't use absolute paths for the diff

pg_regress: Don't use absolute paths for the diff

Don't expand inputfile and outputfile to absolute paths globally, just
where needed.  In particular, pass them as is to the file name
arguments of the diff command, so that we don't see the full absolute
path in the diff header, which makes the diff unnecessarily verbose
and harder to read.

Discussion: https://www.postgresql.org/message-id/0cc82900-c457-1cee-3ab2-7b0f5d215061@2ndquadrant.com