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.
diff --git a/src/testdir/screendump.vim b/src/testdir/screendump.vim
index 50e3384..a147b5a 100644
--- a/src/testdir/screendump.vim
+++ b/src/testdir/screendump.vim
@@ -144,7 +144,7 @@
     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
@@ -152,7 +152,7 @@
 	  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)
diff --git a/src/version.c b/src/version.c
index bf1a1f2..4eb37a7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1556,
+/**/
     1555,
 /**/
     1554,