odb/transaction: introduce ODB transaction flags

odb/transaction: introduce ODB transaction flags

The temporary directory used by git-receive-pack(1) to write objects is
managed slightly differently than how it is done via ODB transactions:

  - The temporary directory is eagerly created upfront, instead of
    waiting for the first object write.

  - The prefix name of the temporary directory is "incoming" instead of
    "bulk-fsync".

In a subsequent commit, git-receive-pack(1) will use ODB transactions
instead of `tmp_objdir` directly. To provide a means to configure the
same transaction behavior, introduce `enum odb_transaction_flags` and
the ODB_TRANSACTION_RECEIVE flag intended as a signal for ODB
transactions using the "files" backend to be set up for
git-receive-pack(1). Transaction call sites are updated accordingly to
provide the required flag parameter.

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