builtin/blame: fix leaking prefixed paths

builtin/blame: fix leaking prefixed paths

In `cmd_blame()` we compute prefixed paths by calling `add_prefix()`,
which itself calls `prefix_path()`. While `prefix_path()` returns an
allocated string, `add_prefix()` pretends to return a constant string.
Consequently, this path never gets freed.

Fix the return type to be `char *` and free the path to plug the memory
leak.

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