object-file: rename files transaction prepare function

object-file: rename files transaction prepare function

The "files" ODB transaction backend lazily creates a temporary object
directory when the first loose object is written to the transaction via
`prepare_loose_object_transaction()`. In a subsequent commit, the
temporary directory is used to also write packfiles to.

Rename the function to `odb_transaction_files_prepare()` accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>