commit.c: add repo_get_commit_tree()

commit.c: add repo_get_commit_tree()

Remove the implicit dependency on the_repository in this function.
It will be used in sha1-name.c functions when they are updated to take
any 'struct repository'. get_commit_tree() remains as a compat wrapper,
to be slowly replaced later.

Any access to "maybe_tree" field directly will result in _broken_ code
after running through commit.cocci because we can't know what is the
right repository to use.

the_repository would be correct most of the time. But we're relying less
and less on the_repository and that assumption may no longer be
true. The transformation now is more of a poor man replacement for a C++
compiler catching access to private fields.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>