Trimming optional in completing-read-multiple

[Mosè Giordano]

Hi all,

this commit 
http://git.savannah.gnu.org/gitweb/?p=emacs.git;a=commitdiff;h=9c44569ea2a18099307e0571d523d8637000a153
made `completing-read-multiple' discard empty strings in its return
value, but AUCTeX has at least one function relying on empty strings
in `completing-read-multiple' output, see
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19504

Would it be possible to make trimming optional (off by default), using
an optional argument?  In AUCTeX we support all Emacs versions
starting from 21, so a new mandatory argument wouldn't be a good idea
for us.

Bye,
Mosè

PS please keep me in CC, I'm not subscribed to the list