ob-tangle: Restore handling of relative file links

* lisp/ob-tangle.el (org-babel-tangle-single-block): Fix regression
from v9.0.2 that resulted in org-babel-tangle-use-relative-file-links
being ignored.
* testing/lisp/test-ob-tangle.el
(ob-tangle/comment-links-relative-file): Add test.

As of 7b148e2d0 (ob-tangle: Respect buffer local variables,
2016-12-10), the condition in org-babel-tangle-single-block that
checks whether to return a file link with a relative target always
returns nil because the full org-link-types-re match, including the
final colon, is compared with "file" (no colon).  Also, were this
condition to succeed, "file:" would be used as the file name.

Adjust the condition and fix the extracted file name.

Reported-by: Jeremias Gonzalez <jgonzalez49@ucmerced.edu>
https://orgmode.org/list/6fb12326-52d3-7177-eff5-62603261b388@ucmerced.edu

lisp/ob-tangle.el

@@ -471,9 +471,9 @@ non-nil, return the full association list to be used by
 		  file)
 		(if (and org-babel-tangle-use-relative-file-links
 			 (string-match org-link-types-re link)
-			 (string= (match-string 0 link) "file"))
+			 (string= (match-string 1 link) "file"))
 		    (concat "file:"
-			    (file-relative-name (match-string 1 link)
+			    (file-relative-name (substring link (match-end 0))
 						(file-name-directory
 						 (cdr (assq :tangle params)))))
 		  link)

testing/lisp/test-ob-tangle.el

@@ -120,6 +120,43 @@ echo 1
                   (search-forward "[H:2]]" nil t))))
        (delete-file "test-ob-tangle.el")))))
 
+(ert-deftest ob-tangle/comment-links-relative-file ()
+  "Test relative file name handling when commenting with links."
+  (should
+   (org-test-with-temp-text-in-file
+       "* H
+#+header: :tangle \"test-ob-tangle.el\" :comments link
+#+begin_src emacs-lisp
+1
+#+end_src"
+     (unwind-protect
+	 (let ((org-babel-tangle-use-relative-file-links t))
+	   (org-babel-tangle)
+	   (with-temp-buffer
+	     (insert-file-contents "test-ob-tangle.el")
+	     (buffer-string)
+	     (goto-char (point-min))
+	     (search-forward
+	      (concat "[file:" (file-name-nondirectory file))
+	      nil t)))
+       (delete-file "test-ob-tangle.el"))))
+  (should
+   (org-test-with-temp-text-in-file
+       "* H
+#+header: :tangle \"test-ob-tangle.el\" :comments link
+#+begin_src emacs-lisp
+1
+#+end_src"
+     (unwind-protect
+	 (let ((org-babel-tangle-use-relative-file-links nil))
+	   (org-babel-tangle)
+	   (with-temp-buffer
+	     (insert-file-contents "test-ob-tangle.el")
+	     (buffer-string)
+	     (goto-char (point-min))
+	     (search-forward (concat "[file:" file) nil t)))
+       (delete-file "test-ob-tangle.el")))))
+
 (ert-deftest ob-tangle/jump-to-org ()
   "Test `org-babel-tangle-jump-to-org' specifications."
   ;; Standard test.