make src block location available to execution backends

* lisp/ob-core.el (org-babel-where-is-src-block-head): return
  point-marker instead of point.

  (org-babel-current-exec-src-block-head): new global variable which is
  bound during `org-babel-execute-src-block-maybe'

  (org-babel-get-src-block-info): return a list of 7 elements, last
  being the header location

Evaluation backends might need to use the exact location of the block in
original org file. For example when inserting the correct source
references for visual debugging. With this patch the information on
current block is available in `org-babel-current-exec-src-block-head'.

lisp/ob-core.el

@@ -251,14 +251,14 @@ references; a process which could likely result in the execution
 of other code blocks.
 
 Returns a list
- (language body header-arguments-alist switches name indent)."
+ (language body header-arguments-alist switches name indent block-head)."
   (let ((case-fold-search t) head info name indent)
     ;; full code block
     (if (setq head (org-babel-where-is-src-block-head))
 	(save-excursion
 	  (goto-char head)
 	  (setq info (org-babel-parse-src-block-match))
-	  (setq indent (car (last info)))
+	  (setq indent (nth 5 info))
 	  (setq info (butlast info))
 	  (while (and (forward-line -1)
 		      (looking-at org-babel-multi-line-header-regexp))
@@ -274,7 +274,7 @@ Returns a list
     ;; resolve variable references and add summary parameters
     (when (and info (not light))
       (setf (nth 2 info) (org-babel-process-params (nth 2 info))))
-    (when info (append info (list name indent)))))
+    (when info (append info (list name indent head)))))
 
 (defvar org-current-export-file) ; dynamically bound
 (defmacro org-babel-check-confirm-evaluate (info &rest body)
@@ -535,6 +535,8 @@ can not be resolved.")
 
 ;;; functions
 (defvar call-process-region)
+(defvar org-babel-current-exec-src-block-head nil
+  "Marker to the currently processed src block.")
 
 ;;;###autoload
 (defun org-babel-execute-src-block (&optional arg info params)
@@ -562,6 +564,7 @@ block."
       (let* ((params (if params
 			 (org-babel-process-params merged-params)
 		       (nth 2 info)))
+	     (org-babel-current-exec-src-block-head (nth 6 info))
 	     (cachep (and (not arg) (cdr (assoc :cache params))
 			   (string= "yes" (cdr (assoc :cache params)))))
 	     (new-hash (when cachep (org-babel-sha1-hash info)))
@@ -592,7 +595,7 @@ block."
 		  (or (org-bound-and-true-p
 		       org-babel-call-process-region-original)
 		      (symbol-function 'call-process-region)))
-		 (indent (car (last info)))
+		 (indent (nth 5 info))
 		 result cmd)
 	    (unwind-protect
 		(let ((call-process-region
@@ -1592,7 +1595,7 @@ If the point is not on a source block then return nil."
         (< top initial) (< initial bottom)
         (progn (goto-char top) (beginning-of-line 1)
 	       (looking-at org-babel-src-block-regexp))
-        (point))))))
+        (point-marker))))))
 
 ;;;###autoload
 (defun org-babel-goto-src-block-head ()