Diagnose logs incorrect cache server url

[jrbriggs]

Steps to reproduce:

1. Have a cache server.
2. Run gvfs diagnose.
3. Look in diagnostics.log

Expected Behavior:

1. Should have correct cache server url.

Instead shows invalid://repoUrl.

[derrickstolee]

This also affects jobs like gvfs repair and can even lead to the URL being passed to a credential manager.

[jeschu1]

There are 2 different bugs here

1. Under certain conditions we don't correctly init the enlistment on repair, causing us to use an invalid url. This is do to a miss on refactoring.
2. If we don't have a cache server we are showing an invalid url instead of nothing. This is due to us using the default url since it wasn't overwritten on the config lookup.

I will addresss each in separate PRs.