bug#17899: 24.3.92.1; dabbrev-completion incorrectly searches all buffers

[Stefan Guath]

VERSION:
GNU Emacs 24.3.92.1 (x86_64-apple-darwin13.2.0, NS apple-appkit-1265.20)

TO REPRODUCE BUG:
* emacs -Q
* Create two buffers (C-x b) with the following contents:
foo.el: "idris0 idris1"
bar.el: "ideal0 ideal1"
* Go to end of foo.el, write "id" and execute dabbrev-completion (C-M-/)

EXPECTED RESULT
"id" should complete to "idris". The doc of dabbrev-completion is very clear on 
this:

"Like M-/ but finds all expansions in the current buffer
and presents suggestions for completion.

With a prefix argument ARG, it searches all buffers accepted by the
function pointed out by `dabbrev-friend-buffer-function' to find the
completions.

If the prefix argument is 16 (which comes from C-u C-u),
then it searches *all* buffers."

No prefix was given here, so it should just look in the current buffer.

ACTUAL RESULT
A new buffer opens with the text:

"In this buffer, type RET to select the completion near point.

Possible completions are:
ideal0                                 ideal1
idris0                                 idris1"

Apparently, the buffer bar.el was incorrectly taken into account. After killing 
buffer bar.el, dabbrev-completion completes as expected in foo.el.