grep: do not segfault when -f is used "git grep" would segfault if its -f option was used because it would try to use an uninitialized strbuf, so initialize the strbuf. Thanks to Johannes Sixt <j.sixt@viscovery.net> for the help with the test cases. Signed-off-by: Matt Kraai <kraai@ftbfs.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>