Create explain_format.c and move relevant code there.

explain.c has grown rather large, so move various functions that
are principally concerned with output generation to a new source
file, explain_format.c, instead of lumping them in with everything
else that is part of explain.c

Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: http://postgr.es/m/CA+TgmoYutMw1Jgo8BWUmB3TqnOhsEAJiYO=rOQufF4gPLWmkLQ@mail.gmail.com

Author: robertmhaas

contrib/auto_explain/auto_explain.c

@@ -16,6 +16,7 @@
 
 #include "access/parallel.h"
 #include "commands/explain.h"
+#include "commands/explain_format.h"
 #include "common/pg_prng.h"
 #include "executor/instrument.h"
 #include "utils/guc.h"

contrib/file_fdw/file_fdw.c

@@ -25,6 +25,7 @@
 #include "commands/copyfrom_internal.h"
 #include "commands/defrem.h"
 #include "commands/explain.h"
+#include "commands/explain_format.h"
 #include "commands/vacuum.h"
 #include "foreign/fdwapi.h"
 #include "foreign/foreign.h"

contrib/postgres_fdw/postgres_fdw.c

@@ -20,6 +20,7 @@
 #include "catalog/pg_opfamily.h"
 #include "commands/defrem.h"
 #include "commands/explain.h"
+#include "commands/explain_format.h"
 #include "executor/execAsync.h"
 #include "foreign/fdwapi.h"
 #include "funcapi.h"

src/backend/commands/Makefile

@@ -34,6 +34,7 @@ OBJS = \
 	dropcmds.o \
 	event_trigger.o \
 	explain.o \
+	explain_format.o \
 	extension.o \
 	foreigncmds.o \
 	functioncmds.o \