repository: introduce raw object store field

repository: introduce raw object store field

The raw object store field will contain any objects needed for access
to objects in a given repository.

This patch introduces the raw object store and populates it with the
`objectdir`, which used to be part of the repository struct.

As the struct gains members, we'll also populate the function to clear
the memory for these members.

In a later step, we'll introduce a struct object_parser, that will
complement the object parsing in a repository struct: The raw object
parser is the layer that will provide access to raw object content,
while the higher level object parser code will parse raw objects and
keeps track of parenthood and other object relationships using 'struct
object'.  For now only add the lower level to the repository struct.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>