source block variable expansion

[Rick Frankel <rick@rickster.com>]

Since org-mode v7.8, editing a code block in an indirect buffer causes
any referenced code blocks to be executed. While this behavior is
desired for viewing code in an indirect buffer (as the behavior has
always been) it is not for editing (esp. if the named block takes a
long time to run).

Given a referenced source block:

#+name: var
#+begin_src elisp
  "you shouldn't see this in the mini-buffer"
#+end_src

When editing the followiing code block (via =C-c '=), the above block
(=var=) will be executed. The message =executing Elisp code block
(var)...= will appear in the =*Message*= buffer and the message above
will appear in the mini-buffer.

#+name: edit-test(var=var)
#+begin_src perl
  $var;
#+end_src