Some implementations do not know what to do with -H; define
NO_H_OPTION_IN_GREP when you build git if your grep lacks -H.

Most of the time, it can be worked around by prepending
/dev/null to the argument list, but that causes -L and -c to
slightly misbehave (they both expose /dev/null is given), so
when these options are given, do not run external grep that does
not understand -H.
Signed-off-by: NJunio C Hamano <junkio@cox.net>