[BUG] Formatting of bracketed numbers in included source files

[Kyle Meyer <kyle@kyleam.com>]

Hello,

I'm experiencing issues including a file during export if that file has
a single number surrounded by brackets.

,----[ test.org ]
| #+include: "test.py" src Python
`----

,----[ test.py ]
| x = [1]
`----

With the current master (51893a4), trying to export test.org leads to
the following error.

,----
| Debugger entered--Lisp error: (error "Format specifier doesn't match argument type")
|   format("fn:%d-" "nil")
|   org-export--prepare-file-contents("/tmp/test.py" nil)
|   org-export-expand-include-keyword()
|   org-export-as(ascii nil nil nil (:ascii-charset ascii))
|   org-export-to-buffer(ascii "*Org ASCII Export*" nil nil nil nil (:ascii-charset ascii) #[nil "\300 \207" [text-mode] 1])
|   org-ascii-export-as-ascii(nil nil nil nil (:ascii-charset ascii))
|   (lambda (a s v b) (org-ascii-export-as-ascii a s v b (quote (:ascii-charset ascii))))(nil nil nil nil)
|   org-export-dispatch(nil)
|   ad-Orig-call-interactively(org-export-dispatch nil nil)
|   call-interactively(org-export-dispatch nil nil)
`----

It seems that `org-export--prepare-file-contents' is treating "[1]" as a
footnote. Bisecting this error suggests commit
b8781c4c85f0d51cecb32e5192b5049e8f241939 introduced the issue.

--
Kyle