cache-tree: allow writing in-memory index as tree

cache-tree: allow writing in-memory index as tree

The function `write_in_core_index_as_tree()` takes a repository and
writes its index into a tree object. What this function cannot do though
is to take an _arbitrary_ in-memory index.

Introduce a new `struct index_state` parameter so that the caller can
pass a different index than the one belonging to the repository. This
will be used in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>