Don't serialize internal hash collision chain link

Don't serialize internal hash collision chain link

ObjectId is serializable, and so are its subtypes. Ensure that
serialization does not follow the hash collision chain internal to the
ObjectIdOwnerMap, otherwise completely unrelated objects may get
serialized when a RevObject is serialized.

Note that serializing a RevCommit or RevTag may serialize quite a few
objects due to the parent/object links they contain. A user has no real
control over how many objects will be written when a RevCommit is
serialized. C.f [1]. This change does not resolve that, but in any case
this internal hash collision chain link should not participate in
serialization.

[1] https://github.com/gitblit/gitblit/pull/1141

Change-Id: Ice331a9dc80a59ca360fcc04adaff8b5e750d847
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>