Currently libgit2 shares pointers to its internal reference cache with
the user. This leads to several problems like invalidation of reference
pointers when reordering the cache or manipulation of the cache from
user side.

Give each user its own git_reference instead of leaking the internal
representation (struct reference).

Add the following new API functions:

	* git_reference_free
	* git_reference_is_packed

Signed-off-by: schu <schu-github@schulog.org>