Tests create global config stub if required

Instead of relying on the global configuration and skipping the test if
Git is not installed, the following tests create a configuration file
containing user.name and user.email using
BaseFixture.CreateConfigurationWithDummyUser(Signature):

* CommitFixture.CanCommitWithSignatureFromConfig()
* ConfigurationFixture.CanEnumerateGlobalConfig()

Fixes #561
Enforce Network.ListReferences() test coverage

This puts under the light that DirectReferences may point to GitObjects unknown from the local repository.
Improve Network.ListReferences() documentation
Enumerate refspecs

The Remote.RefSpecs property allows to enumerate over all refspecs defined for a specific remote.