org-babel-detangle: Fix :comments link for src blocks before first heading

* lisp/ob-tangle.el (org-babel-tangle-jump-to-org): Allow fuzzy
matching when following the backlink to src block from tangles
source.  The current default value of
`org-link-search-must-match-exact-headline` makes
`org-link-open-from-string` treat fuzzy links as headlines without
falling back to fuzzy text search.  Bind it to nil to force fuzzy text
matching while detangling.

Fixes https://orgmode.org/list/87zgh1qxfk.fsf@posteo.net

lisp/ob-tangle.el

@@ -581,7 +581,10 @@ which enable the original code blocks to be found."
 	  (error "Not in tangled code"))
         (setq body (buffer-substring body-start end)))
       ;; Go to the beginning of the relative block in Org file.
-      (org-link-open-from-string link)
+      ;; Explicitly allow fuzzy search even if user customized
+      ;; otherwise.
+      (let (org-link-search-must-match-exact-headline)
+        (org-link-open-from-string link))
       (setq target-buffer (current-buffer))
       (if (string-match "[^ \t\n\r]:\\([[:digit:]]+\\)" block-name)
           (let ((n (string-to-number (match-string 1 block-name))))