merge-tree: only use basic merge config

merge-tree: only use basic merge config

Commit 9c93ba4d0ae (merge-recursive: honor diff.algorithm, 2024-07-13)
replaced init_merge_options() with init_basic_merge_config() for use in
plumbing commands and init_ui_merge_config() for use in porcelain
commands. As "git merge-tree" is a plumbing command it should call
init_basic_merge_config() rather than init_ui_merge_config(). The merge
ort machinery ignores "diff.algorithm" so the behavior is unchanged by
this commit but it future proofs us against any future changes to
init_ui_merge_config().

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>