Add CLI option to get a graph of all tracked files

[devdanzin]

Using wily programmatically it's possible to call graph() with an empty path to get a graph that includes all tracked files. It could be useful to expose this feature in the CLI.

Describe the solution you'd like
Add a simple switch to get a graph of all tracked files.

Describe alternatives you've considered
Maybe it could also be done by adding a switch that takes path explicitly and allows it to be empty, but it seems unnecessarily complex.

While I don't currently have code for the CLI option, it should be easy to add and I can send a PR if it's desirable. It might be better to have other graph PRs resolved first.