store repository URL in rooted repositories

[smola]

URL which was used to fetch a repository should be present in rooted repositories. We can do this by leveraging git remotes config.

The result would be:

• For each remote repository, there is a remote with the repository ULID as its remote name.
• We would not create special names for the references like now, ULID would be just the remote name in reference names like refs/remotes/<ULID>/<branch>

[smola]

Depends on src-d/go-git#501 if we want to store all endpoints.

[smola]

PR: #120