Added more extensive git reference test.
Added better error checking for git revision walker.
Added missing dependency on asdf.
Removed duplicate entry for git_reflog_entry_committer.
Added tests for reference accessors.
Re-factored reference tests to use a fixture.
Added reference target test..
Extended references tests to cover almost every part of the reference API.

* Updated test repository macro so that it can be used to create bare repositories.
* Added error when trying to find the target of a symbolic reference.