builtin/blame: fix leaking ignore revs files

builtin/blame: fix leaking ignore revs files

When parsing the blame configuration we add "blame.ignoreRevsFile"
configs to a string list. This string list is declared as with `NODUP`,
and thus we hand over the allocated string to that list. We eventually
end up calling `string_list_clear()` on that list, but due to it being
declared as `NODUP` we will not release the associated strings and thus
leak memory.

Fix this issue by setting up the list as `DUP` instead and free the
config string after insertion.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>