grep: unset GREP_OPTIONS before spawning external grep

While we're at it, also unset GREP_COLOR and GREP_COLORS in case colouring
is not enabled, to be on the safe side.  The presence of these variables
alone is not sufficient to trigger coloured output with GNU grep, but
other implementations may behave differently.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

diff --git a/builtin-grep.c b/builtin-grep.c
index 01be9bf..9a9e3fc 100644
--- a/builtin-grep.c
+++ b/builtin-grep.c
@@ -433,7 +433,11 @@
 
 		if (opt->color_external && strlen(opt->color_external) > 0)
 			push_arg(opt->color_external);
+	} else {
+		unsetenv("GREP_COLOR");
+		unsetenv("GREP_COLORS");
 	}
+	unsetenv("GREP_OPTIONS");
 
 	hit = 0;
 	argc = nr;

t/t7002-grep.sh

diff --git a/t/t7002-grep.sh b/t/t7002-grep.sh
index ae5290a..dd0da6c 100755
--- a/t/t7002-grep.sh
+++ b/t/t7002-grep.sh
@@ -213,6 +213,11 @@
 	test_cmp expected actual
 '
 
+test_expect_success 'grep should ignore GREP_OPTIONS' '
+	GREP_OPTIONS=-v git grep " mmap bar\$" >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'grep -f, non-existent file' '
 	test_must_fail git grep -f patterns
 '