(comint-exec-1): Set EMACS and INSIDE_EMACS to t.

lisp/comint.el

@@ -769,7 +769,8 @@ buffer.  The hook `comint-exec-hook' is run after each exec."
 	    (list "TERM=emacs"
 		  (format "TERMCAP=emacs:co#%d:tc=unknown:" (window-width))))
 	  (unless (getenv "EMACS")
-	    (list (concat "EMACS=" invocation-directory invocation-name)))
+	    (list "EMACS=t"))
+	  (list "INSIDE_EMACS=t")
 	  process-environment))
 	(default-directory
 	  (if (file-accessible-directory-p default-directory)