patch 9.0.0306: buffer write message is two lines in message popup window

Problem:    Buffer write message is two lines in message popup window.
Solution:   Overwrite message if "msg_scroll" is off.
diff --git a/src/testdir/dumps/Test_cmdheight_zero_5.dump b/src/testdir/dumps/Test_cmdheight_zero_5.dump
new file mode 100644
index 0000000..dd64210
--- /dev/null
+++ b/src/testdir/dumps/Test_cmdheight_zero_5.dump
@@ -0,0 +1,6 @@
+|s+0&#ffffff0|o|m|e| >t|e|x|t| @65
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|═+0#e000002&@74
+|"|X|s|o|m|e|T|e|x|t|"| |[|N|e|w|]| |1|L|,| |1|0|B| |w|r|i|t@1|e|n| @41
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 7c71edc..f714478 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -494,13 +494,17 @@
   call VerifyScreenDump(buf, 'Test_cmdheight_zero_3', {})
 
   " Message for CTRL-C is in the popup window
-  "call term_sendkeys(buf, ":call popup_clear()\<CR>")
   call term_sendkeys(buf, "\<C-C>")
   call VerifyScreenDump(buf, 'Test_cmdheight_zero_4', {})
 
+  " file write message is one line
+  call term_sendkeys(buf, ":w XsomeText\<CR>")
+  call VerifyScreenDump(buf, 'Test_cmdheight_zero_5', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestCmdheight')
+  call delete('XsomeText')
 endfunc
 
 func Test_cmdheight_zero_shell()