Keep metadata visible when cycling visibility

Metadata blocks like the following appear to contain setext headers when
using regexp matching (as `outline-minor-mode` does).

    ---
    title = Post Name
    date = May 6, 2016 11:15 EDT
    ---

As a result, when cycling visibility of sections the last line of the
block would be hidden.  This patch adds code to detect and revert this
in `markdown-outline-fix-visibility`.

Closes GH-136.

markdown-mode.el

@@ -5563,9 +5563,21 @@ Derived from `org-end-of-subtree'."
 (defun markdown-outline-fix-visibility ()
   "Hide any false positive headings that should not be shown.
 For example, headings inside preformatted code blocks may match
-`outline-regexp' but should not be shown as headings when cycling."
+`outline-regexp' but should not be shown as headings when cycling.
+Also, the ending --- line in metadata blocks appears to be a
+setext header, but should not be folded."
   (save-excursion
     (goto-char (point-min))
+    ;; Unhide any false positives in metadata blocks
+    (when (markdown-text-property-at-point 'markdown-yaml-metadata-begin)
+      (let* ((body (progn (forward-line)
+                          (markdown-text-property-at-point
+                           'markdown-yaml-metadata-section)))
+             (end (progn (goto-char (cl-second body))
+                         (markdown-text-property-at-point
+                          'markdown-yaml-metadata-end))))
+        (outline-flag-region (point-min) (1+ (cl-second end)) nil)))
+    ;; Hide any false positives in code blocks
     (unless (outline-on-heading-p)
       (outline-next-visible-heading 1))
     (while (< (point) (point-max))

tests/markdown-test.el

@@ -3203,6 +3203,21 @@ x: x
      (should (eq (point) 159))
      (should (looking-at "^# Level one again")))))
 
+(ert-deftest test-markdown-outline/visibility-with-metadata ()
+  "Test outline visibility cycling with metadata blocks."
+  (markdown-test-string
+   "---
+layout = post
+date = 2015-08-13 11:35:25 EST
+---
+"
+   (let (last-command this-command)
+     ;; Cycle global visibility to "overview" mode
+     (setq this-command 'markdown-cycle)
+     (markdown-cycle t)
+     ;; Check that text is visible
+     (markdown-test-range-has-property (point-min) (point-max) 'invisible nil))))
+
 ;;; Movement tests:
 
 (ert-deftest test-markdown-movement/defun ()