bug#24941: Early termination bug in grep 2.26

[Gary Johnson]

There was some recent discussion on the vim_dev list of a failure to
update a Vim package which was found to be due to an update of grep
from 2.25 to 2.26.  The details of the grep behavior are discussed
here:

https://www.linuxquestions.org/questions/slackware-14/pkgtools-grep-bug-in-slackware[64]-current-4175593054/

In short, it seems to be due to the "grep: /dev/null output
speedup" commit of 2016-05-01, af6af288eac28951b5eee1eaaf373e22b2193b7b.
When grep terminates early, it closes the pipe it's reading stdin
from, which terminates the program on the other side of that pipe
early, before that program has completed its task.

Oops.

Regards,
Gary