Refactor Refs to preserve their target objects instead of just a string representation

Notice that in spec/tag_spec.rb I changed the sha because previously they referenced the tag annotation commit. With the refactor, the sha must be the one of the commit that the tag actually points at.

Closes #22 (closed)

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/17073

Added 1 commit:

• d481bf4e - Move implementation of local_branches to Gitlab::Git::Repository from gitlab-ce

I bumped the version because the interface for Gitlab::Git::Ref changes.

Status changed to merged

mentioned in commit 33560233

@eReGeBe Release gitlab_git 10.4.0!

@DouweM thanks!

Mentioned in issue #30 (closed)

Mentioned in commit 43f5700b