Allow using refs for project local links

Sphinx is using refs to project internal targets in other files. This
patch convert org mode file links to such refs. Enabled via setting.

ox-rst.el

@@ -98,6 +98,7 @@
     (:rst-link-home "RST_LINK_HOME" nil org-rst-link-home)
     (:rst-link-use-ref-role nil nil org-rst-link-use-ref-role)
     (:rst-extension nil nil org-rst-extension)
+    (:rst-file-link-use-ref-role nil nil org-rst-file-link-use-ref-role)
     (:rst-text-markup-alist nil nil org-rst-text-markup-alist)
     (:rst-quote-margin nil nil org-rst-quote-margin)
     (:rst-headline-underline-characters nil nil org-rst-headline-underline-characters)
@@ -185,6 +186,11 @@ link's path."
   :group 'org-export-rst
   :type 'boolean)
 
+(defcustom org-rst-file-link-use-ref-role nil
+  "Non-nil means export internal file links using :ref: role."
+  :group 'org-export-rst
+  :type 'boolean)
+
 (defcustom org-rst-text-markup-alist '((bold . "**%s**")
 									   (code . verb)
 									   (italic . "*%s*")
@@ -947,6 +953,7 @@ INFO is a plist holding contextual information."
 					   (plist-get info :rst-extension)))
 			  (t raw-path)))))
 		 (type (org-element-property :type link))
+		 (search-option (org-element-property :search-option link))
 		 (raw-path (org-element-property :path link))
 		 ;; Ensure DESC really exists, or set it to nil.
 		 (desc (and (not (string= desc "")) desc))
@@ -1050,6 +1057,13 @@ INFO is a plist holding contextual information."
      ((string= type "coderef")
       (format (org-export-get-coderef-format path desc)
 			  (org-export-resolve-coderef path info)))
+     ((and (plist-get info :rst-file-link-use-ref-role)
+           (string= type "file")
+           search-option)
+      (let ((ref (replace-regexp-in-string "^#" "" search-option)))
+        (if desc
+            (format ":ref:`%s <%s>`" desc ref)
+          (format ":ref:`%s`" ref))))
      ;; Link type is handled by a special function.
      ;((functionp (setq protocol (nth 2 (assoc type org-link-protocols))))
      ; (funcall protocol (org-link-unescape path) desc 'latex))