pg_rewind: Don't error if the two clusters are already on the same timeline

This previously resulted in an error and a nonzero exit status, but
after discussion this should rather be a noop with a zero exit status.

src/bin/pg_rewind/pg_rewind.c

@@ -216,8 +216,12 @@ main(int argc, char **argv)
 	 * do.
 	 */
 	if (ControlFile_target.checkPointCopy.ThisTimeLineID == ControlFile_source.checkPointCopy.ThisTimeLineID)
-		pg_fatal("source and target cluster are on the same timeline\n");
-
+	{
+		printf(_("source and target cluster are on the same timeline\n"));
+		rewind_needed = false;
+	}
+	else
+	{
 		findCommonAncestorTimeline(&divergerec, &lastcommontliIndex);
 		printf(_("servers diverged at WAL position %X/%X on timeline %u\n"),
 			   (uint32) (divergerec >> 32), (uint32) divergerec,
@@ -252,6 +256,7 @@ main(int argc, char **argv)
 			else
 				rewind_needed = true;
 		}
+	}
 
 	if (!rewind_needed)
 	{

src/bin/pg_rewind/t/005_same_timeline.pl

+use strict;
+use warnings;
+use TestLib;
+use Test::More tests => 1;
+
+use RewindTest;
+
+# Test that running pg_rewind if the two clusters are on the same
+# timeline runs successfully.
+
+RewindTest::setup_cluster();
+RewindTest::start_master();
+RewindTest::create_standby();
+RewindTest::run_pg_rewind('local');