If rsync is put in the background, output fewer progress-report

lines.

progress.c

@@ -115,10 +115,24 @@ void end_progress(OFF_T size)
 void show_progress(OFF_T ofs, OFF_T size)
 {
 	struct timeval now;
+#if HAVE_GETPGRP && HAVE_TCGETPGRP
+	static pid_t pgrp = -1;
+	pid_t tc_pgrp;
+#endif
 
 	if (am_server)
 		return;
 
+#if HAVE_GETPGRP && HAVE_TCGETPGRP
+	if (pgrp == -1) {
+# if GETPGRP_VOID
+		pgrp = getpgrp();
+# else
+		pgrp = getpgrp(0);
+# endif
+	}
+#endif
+
 	gettimeofday(&now, NULL);
 
 	if (!ph_start.time.tv_sec) {
@@ -150,5 +164,11 @@ void show_progress(OFF_T ofs, OFF_T size)
 		ph_list[newest_hpos].ofs = ofs;
 	}
 
+#if HAVE_GETPGRP && HAVE_TCGETPGRP
+	tc_pgrp = tcgetpgrp(STDOUT_FILENO);
+	if (tc_pgrp != pgrp && tc_pgrp != -1)
+		return;
+#endif
+
 	rprint_progress(ofs, size, &now, False);
 }