patch 8.1.1523: cannot show range of buffer lines in popup window

Problem:    Cannot show range of buffer lines in popup window.
Solution:   Add the "firstline" property. (closes #4523)
diff --git a/src/testdir/dumps/Test_popupwin_firstline.dump b/src/testdir/dumps/Test_popupwin_firstline.dump
new file mode 100644
index 0000000..276f4e5
--- /dev/null
+++ b/src/testdir/dumps/Test_popupwin_firstline.dump
@@ -0,0 +1,10 @@
+>1+0&#ffffff0| @73
+|2| @73
+|3| @73
+|4| @33|3+0#0000001#ffd7ff255@4| +0#0000000#ffffff0@34
+|5| @33|4+0#0000001#ffd7ff255@1| @2| +0#0000000#ffffff0@34
+|6| @33|5+0#0000001#ffd7ff255| @3| +0#0000000#ffffff0@34
+|7| @33|6+0#0000001#ffd7ff255@4| +0#0000000#ffffff0@34
+|8| @73
+|9| @73
+@57|1|,|1| @10|T|o|p|