patch 9.1.1537: helptoc: still some issues when markdown code blocks

Problem:  helptoc: still some issues when parsing markdown code blocks
          (VimWei)
Solution: assign nextline to curline before starting the next loop
          iteration; when processing fenced code blocks, flip the
          skip_fence variable, update the test to be more comprehensive

related: https://github.com/vim/vim/issues/17699#issuecomment-3055603968
closes: #17716

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -719,6 +719,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1537,
 /**/
     1536,
 /**/