Rename `git_merge_trees()` to `unpack_trees_start()` and extract the
call to `discard_index()` into a new function `unpack_trees_finish()`.
As a result, these are called early resp. late in `merge_trees()`,
making the resource handling clearer. A later commit will expand on
that, teaching `..._finish()` to free more memory. (So rather than
moving the FIXME-comment, just drop it, since it will be addressed soon
enough.)
Also call `..._finish()` when `merge_trees()` returns early.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Martin Ă…gren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>