patch 8.1.1556: command for failing screenshot does not include directory

Problem:    The command displayed to show a failing screenshot does not include
            the "testdir" directory.
Solution:   Prefix the directory name so that it can be copy-pasted.

Author: brammool

src/testdir/screendump.vim

@@ -144,15 +144,15 @@ func VerifyScreenDump(buf, filename, options, ...)
     if i == 100
       " Leave the failed dump around for inspection.
       if filereadable(reference)
-	let msg = 'See dump file difference: call term_dumpdiff("' . testfile . '", "' . reference . '")'
+	let msg = 'See dump file difference: call term_dumpdiff("testdir/' .. testfile .. '", "testdir/' .. reference .. '")'
 	if a:0 == 1
 	  let msg = a:1 . ': ' . msg
 	endif
 	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 . '")'
+	let msg = 'See new dump file: call term_dumpload("testdir/' .. testfile .. '")'
       endif
       for i in range(len(refdump))
 	if i >= len(testdump)

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1556,
 /**/
     1555,
 /**/