merge-recursive: remove another implicit dependency on the_repository
Commit
d7cf3a96e9a0 ("merge-recursive.c: remove implicit dependency on
the_repository", 2019-01-12) and follow-ups like commit
34e7771bc644
("Use the right 'struct repository' instead of the_repository",
2019-06-27), removed most implicit uses of the_repository. Convert
calls to get_commit_tree() to instead use repo_get_commit_tree() to get
rid of another.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>