Also restore gpdiff rule to ignore differences in PIDs.

I don't understand why gpdiff didn't pick this rule up from the
src/test/regress/init_file file. But this seems to fix it, so let's get this
in for now, to make the pipeline green.

src/test/isolation2/init_file_isolation2

@@ -3,3 +3,9 @@
 m/^\d+.*gpfaultinjector.*-\[INFO\]:-/
 s/^\d+.*gpfaultinjector.*-\[INFO\]:-//
 -- end_matchignore
+
+-- start_matchsubs
+# entry db matches
+m/\s+\(entry db(.*)+\spid=\d+\)/
+s/\s+\(entry db(.*)+\spid=\d+\)//
+-- end_matchsubs