merge-ort: clean up after failed merge
In
9fefce68dc8 (merge-ort: basic outline for merge_switch_to_result(),
2020-12-13), we added functionality to lay down the result of a merge on
disk. But we forgot to release the data structures in case
`unpack_trees()` failed to run properly.
This was pointed out by the `linux-leaks` job in our CI runs.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>