odb: introduce odb_write_object()

We do not have a backend-agnostic way to write objects into an object
database. While there is `write_object_file()`, this function is rather
specific to the loose object format.

Introduce `odb_write_object()` to plug this gap. For now, this function
is a simple wrapper around `write_object_file()` and doesn't even use
the passed-in object database yet. This will change in subsequent
commits, where `write_object_file()` is converted so that it works on
top of an `odb_source`. `odb_write_object()` will then become
responsible for deciding which source an object shall be written to.

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

commit.c

@@ -1797,8 +1797,8 @@ int commit_tree_extended(const char *msg, size_t msg_len,
 		compat_oid = &compat_oid_buf;
 	}
 
-	result = write_object_file_flags(buffer.buf, buffer.len, OBJ_COMMIT,
-					 ret, compat_oid, 0);
+	result = odb_write_object_ext(the_repository->objects, buffer.buf, buffer.len,
+				      OBJ_COMMIT, ret, compat_oid, 0);
 out:
 	free(parent_buf);
 	strbuf_release(&buffer);