patch 7.4.741
Problem:    When using += with ":set" a trailing comma is not recognized.
            (Issue 365)
Solution:   Don't add a second comma. Add a test. (partly by Christian
            Brabandt)
diff --git a/src/testdir/test_set.ok b/src/testdir/test_set.ok
new file mode 100644
index 0000000..5d6a708
--- /dev/null
+++ b/src/testdir/test_set.ok
@@ -0,0 +1 @@
+*.png,*.jpg