patch 8.1.1129: when making a new screendump test have to create the file

Problem:    When making a new screendump test have to create the file.
Solution:   Continue creating the failed screendump, so it can be moved once
            it is correct.

src/testdir/screendump.vim

@@ -118,7 +118,12 @@ func VerifyScreenDump(buf, filename, options, ...)
     call delete(testfile)
     call term_dumpwrite(a:buf, testfile, a:options)
     let testdump = readfile(testfile)
+    if filereadable(reference)
       let refdump = readfile(reference)
+    else
+      " Must be a new screendump, always fail
+      let refdump = []
+    endif
     if refdump == testdump
       call delete(testfile)
       if did_mkdir
@@ -127,7 +132,8 @@ func VerifyScreenDump(buf, filename, options, ...)
       break
     endif
     if i == 100
-      " Leave the test file around for inspection.
+      " Leave the failed dump around for inspection.
+      if filereadable(reference)
 	let msg = 'See dump file difference: call term_dumpdiff("' . testfile . '", "' . reference . '")'
 	if a:0 == 1
 	  let msg = a:1 . ': ' . msg
@@ -135,6 +141,9 @@ func VerifyScreenDump(buf, filename, options, ...)
 	if len(testdump) != len(refdump)
 	  let msg = msg . '; line count is ' . len(testdump) . ' instead of ' . len(refdump)
 	endif
+      else
+	let msg = 'See new dump file: call term_dumpload("' . testfile . '")'
+      endif
       for i in range(len(refdump))
 	if i >= len(testdump)
 	  break

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1129,
 /**/
     1128,
 /**/