The default grep command for M-x grep.
If the grep program used supports an option to always include file names
in its output (such as the `-H' option to GNU grep), it's a good idea to
include it when specifying `grep-command'.
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'.