pgsql: Use standard diff separator for regression.diffs

Use standard diff separator for regression.diffs

Instead of ======..., use the standard separator for a multi-file
diff, which is, per POSIX,

    "diff %s %s %s\n", <diff_options>, <filename1>, <filename2>

This makes regression.diffs behave more like a proper diff file, for
use with other tools.  And it shows the diff options used, for
clarity.

Discussion: https://www.postgresql.org/message-id/70440c81-37bb-76dd-e48b-b5a9550d5613@2ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8f27a14b1bd3d906144356ce19e33a2fd0095141

Modified Files
--------------
src/test/regress/pg_regress.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)