Fix bug with URL encoding and binder bare Git

[zergar]

Currently, if you use https://mybinder.org/ to launch binder on a standard git repo (i.e., not GitHub or GitLab), in the URL slashes are URL-encoded. This, however, is not reflected in this codebase, where slashes are kept as-is.

This PR sets safe characters to empty in order to also encode slashes in the repository URL.