Fix subtree excerpts

Subtrees should include their initial heading and full body.

org-roam-export.el

@@ -37,10 +37,13 @@
   "Extract an excerpt from ELEMENT in BUFFER."
   (with-current-buffer (or buffer (current-buffer))
     (let ((element (or element (org-element-at-point))))
+      (if (eq 'headline (org-element-type element))
+          (progn (org-mark-subtree)
+                 (buffer-substring (point) (mark)))
         (if-let ((begin (org-element-property :contents-begin element))
                  (end (org-element-property :contents-end element)))
             (buffer-substring begin end)
-        ""))))
+          "")))))
 
 (defun org-roam-export-backlink-excerpt (backlink)
   "Get the Org element containing the link from BACKLINK as an excerpt."

tests.el

@@ -79,18 +79,21 @@ Opening paragraph.
 (ert-deftest excerpt-heading-with-subheadings-test ()
   (with-test-document
    (goto-line 8)
-    (should (equal "  Heading one paragraph.
+    (should (equal "* Heading One
+
+  Heading one paragraph.
 
 ** Subheading One A
 
   - List item 1
   - List item 2
+
 " (org-roam-export--excerpt)))))
 
 (ert-deftest excerpt-simple-heading-test ()
   (with-test-document
    (goto-line 17)
-   (should (equal "  Heading two paragraph.\n" (org-roam-export--excerpt)))))
+   (should (equal "* Heading Two\n\n  Heading two paragraph.\n" (org-roam-export--excerpt)))))
 
 (ert-deftest lorem-backlink-excerpt ()
   (should (equal '("Aliquam [[id:d12a1ce4-3199-42f4-b39b-b68c03458669][lorem]] ante, suscipit a lorem molestie, aliquet elementum eros. Proin