blame: drop strdup of string literal

blame: drop strdup of string literal

This strdup was added as part of 58dbfa2 (blame: accept
multiple -L ranges, 2013-08-06) to be consistent with
parse_opt_string_list(), which appends to the same list.

But as of 7a7a517 (parse_opt_string_list: stop allocating
new strings, 2016-06-13), we should stop using strdup (to
match parse_opt_string_list, and for all the reasons
described in that commit; namely that it does nothing useful
and causes us to leak the memory).

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>