unset GREP_OPTIONS in test-lib.sh

I used to set GREP_OPTIONS to exclude *.orig and *.rej files. But with this
the test t4252-am-options.sh fails because it calls grep with a .rej file:

    grep "@@ -1,3 +1,3 @@" file-2.rej
Signed-off-by: NBert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -65,6 +65,8 @@ GIT_TEST_CMP=${GIT_TEST_CMP:-diff -u}
 # CDPATH into the environment
 unset CDPATH
 
+unset GREP_OPTIONS
+
 case $(echo $GIT_TRACE |tr "[A-Z]" "[a-z]") in
 	1|2|true)
 		echo "* warning: Some tests will not work if GIT_TRACE" \