t7002: set test prerequisite "external-grep" if supported

Add another test to set prerequisite EXTGREP if the current build supports
external grep. This can be used to skip external grep only tests on builds
that do not support this optimization.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7002-grep.sh

@@ -8,6 +8,18 @@ test_description='git grep various.
 
 . ./test-lib.sh
 
+test_expect_success 'Check for external grep support' '
+	case "$(git grep -h 2>&1|grep ext-grep)" in
+	*"(default)"*)
+		test_set_prereq EXTGREP
+		true;;
+	*"(ignored by this build)"*)
+		true;;
+	*)
+		false;;
+	esac
+'
+
 cat >hello.c <<EOF
 #include <stdio.h>
 int main(int argc, const char **argv)