grep-use-null-device

grep-use-null-device is a variable defined in `grep.el'.
Its value is

auto-detect

Documentation:
If t, append the value of `null-device' to `grep' commands.
This is done to ensure that the output of grep includes the filename of
any match in the case where only a single file is searched, and is not
necessary if the grep program used supports the `-H' option.

In interactive usage, the actual value of this variable is set up
by `grep-compute-defaults'; to change the default value, use
Customize or call the function `grep-apply-setting'.

You can customize this variable.