org-babel: remove prompt characters from R output

When evaluating R code in a persistent session, lines of output
typically start with a single prompt character. However, under certain
circumstances, lines start with a concatenation of (primary and
secondary) prompt characters separated by spaces. This seems to be
related to having blank lines in the input. This fix ensures that all
such prompt characters are removed from the output.

contrib/babel/lisp/langs/org-babel-R.el

@@ -155,6 +155,12 @@ last statement in BODY, as elisp."
 		 (mapconcat #'org-babel-chomp (list body org-babel-R-eoe-indicator) "\n"))))
 	     (raw (org-babel-comint-with-output buffer org-babel-R-eoe-output nil
                     (insert full-body) (inferior-ess-send-input)))
+	     (comint-prompt-regexp
+	      (concat "^\\("
+		      inferior-ess-primary-prompt
+		      "\\|"
+		      inferior-ess-secondary-prompt
+		      "\\)*"))
 	     broke results)
         (case result-type
           (value (org-babel-R-process-value-result