Stephane Chazelas wrote:
> 2.25 was OK. git bisect points to commit
> 2769d5331a38d623b67b1860ac46b39ff7e54aca

Thanks for pinpointing the bug. It was my logic error in that commit. Fixed by 
altering Gnulib as follows:

http://lists.gnu.org/archive/html/bug-gnulib/2016-11/msg00086.html

and by installing the attached patches into grep.