bug#20912: 25.0.50; M-x grep no longer works

[Paul Eggert]

The recent commit 9b4b4a8355506a0253d8a4943e0a9aa87f9e92eb broke ordinary use of 
M-x grep.  To reproduce, start emacs -Q and type:

M-x grep RET foo /dev/null RET

The output will say:

grep -nH -e --color foo /dev/null
grep: foo: No such file or directory