Tolerate timeline switches while "pg_basebackup -X fetch" is running.

If you take a base backup from a standby server with "pg_basebackup -X
fetch", and the timeline switches while the backup is being taken, the
backup used to fail with an error "requested WAL segment %s has already
been removed". This is because the server-side code that sends over the
required WAL files would not construct the WAL filename with the correct
timeline after a switch.

Fix that by using readdir() to scan pg_xlog for all the WAL segments in the
range, regardless of timeline.

Also, include all timeline history files in the backup, if taken with
"-X fetch". That fixes another related bug: If a timeline switch happened
just before the backup was initiated in a standby, the WAL segment
containing the initial checkpoint record contains WAL from the older
timeline too. Recovery will not accept that without a timeline history file
that lists the older timeline.

Backpatch to 9.2. Versions prior to that were not affected as you could not
take a base backup from a standby before 9.2.

src/backend/replication/walsender.c

@@ -1170,7 +1170,6 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
 	char	   *p;
 	XLogRecPtr	recptr;
 	Size		nbytes;
-	XLogSegNo	lastRemovedSegNo;
 	XLogSegNo	segno;
 
 retry:
@@ -1263,13 +1262,8 @@ retry:
 	 * read() succeeds in that case, but the data we tried to read might
 	 * already have been overwritten with new WAL records.
 	 */
-	XLogGetLastRemoved(&lastRemovedSegNo);
 	XLByteToSeg(startptr, segno);
-	if (segno <= lastRemovedSegNo)
-		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("requested WAL segment %s has already been removed",
-						XLogFileNameP(sendTimeLine, segno))));
+	CheckXLogRemoved(segno, ThisTimeLineID);
 
 	/*
 	 * During recovery, the currently-open WAL file might be replaced with the