Fix off-by-one error in get_next_line and get_prev_line

The functions stopped at the end/start block marker (respectively)
instead of returning the following line. This caused issues when the
function was subsequently called in markdown mode since the start and
end block markers are the same.

This fixes #407

test/list_update.vader

@@ -0,0 +1,191 @@
+Include: vader_includes/vader_setup.vader
+
+Given vimwiki (Sample nested list, vimwiki syntax):
+  * [ ] Top Level
+    * [ ] Child 1
+    * [ ] Child 2
+
+    * [ ] Post space
+
+    {{{code
+    * [ ] print "hello, world"
+    }}}
+
+    {{{morecode
+    print "hello again"
+    }}}
+
+    * [ ] Post code
+      * [ ] Sub-child
+
+      * [ ] Sub-sub-child
+
+Execute (Set syntax to default):
+  call SetSyntax('default')
+
+Do (Toggle top-level):
+  \<C-Space>
+
+Expect (All tree toggled):
+  * [X] Top Level
+    * [X] Child 1
+    * [X] Child 2
+
+    * [X] Post space
+
+    {{{code
+    * [ ] print "hello, world"
+    }}}
+
+    {{{morecode
+    print "hello again"
+    }}}
+
+    * [X] Post code
+      * [X] Sub-child
+
+      * [X] Sub-sub-child
+
+Do (Toggle child):
+  j
+  \<C-Space>
+
+Expect (Child toggled, top updated):
+  * [.] Top Level
+    * [X] Child 1
+    * [ ] Child 2
+
+    * [ ] Post space
+
+    {{{code
+    * [ ] print "hello, world"
+    }}}
+
+    {{{morecode
+    print "hello again"
+    }}}
+
+    * [ ] Post code
+      * [ ] Sub-child
+
+      * [ ] Sub-sub-child
+
+Do (Toggle sub-child):
+  G
+  \<C-Space>
+
+Expect (Sub-child toggled, parents updated):
+  * [.] Top Level
+    * [ ] Child 1
+    * [ ] Child 2
+
+    * [ ] Post space
+
+    {{{code
+    * [ ] print "hello, world"
+    }}}
+
+    {{{morecode
+    print "hello again"
+    }}}
+
+    * [o] Post code
+      * [ ] Sub-child
+
+      * [X] Sub-sub-child
+
+Given markdown (Sample nested list, markdown syntax):
+  * [ ] Top Level
+    * [ ] Child 1
+    * [ ] Child 2
+
+    * [ ] Post space
+
+    ```code
+    * [ ] print "hello, world"
+    ```
+
+    ```morecode
+    print "hello again"
+    ```
+
+    * [ ] Post code
+      * [ ] Sub-child
+
+      * [ ] Sub-sub-child
+
+Execute (Set syntax to markdown):
+  call SetSyntax('markdown')
+
+Do (Toggle top-level):
+  \<C-Space>
+
+Expect (All tree toggled):
+  * [X] Top Level
+    * [X] Child 1
+    * [X] Child 2
+
+    * [X] Post space
+
+    ```code
+    * [ ] print "hello, world"
+    ```
+
+    ```morecode
+    print "hello again"
+    ```
+
+    * [X] Post code
+      * [X] Sub-child
+
+      * [X] Sub-sub-child
+
+Do (Toggle child):
+  j
+  \<C-Space>
+
+Expect (Child toggled, top updated):
+  * [.] Top Level
+    * [X] Child 1
+    * [ ] Child 2
+
+    * [ ] Post space
+
+    ```code
+    * [ ] print "hello, world"
+    ```
+
+    ```morecode
+    print "hello again"
+    ```
+
+    * [ ] Post code
+      * [ ] Sub-child
+
+      * [ ] Sub-sub-child
+
+Do (Toggle sub-child):
+  G
+  \<C-Space>
+
+Expect (Sub-child toggled, parents updated):
+  * [.] Top Level
+    * [ ] Child 1
+    * [ ] Child 2
+
+    * [ ] Post space
+
+    ```code
+    * [ ] print "hello, world"
+    ```
+
+    ```morecode
+    print "hello again"
+    ```
+
+    * [o] Post code
+      * [ ] Sub-child
+
+      * [X] Sub-sub-child
+
+Include: vader_includes/vader_teardown.vader