repo: unconditionally create a global config backend

When a repository is initialised, we need to probe to see if there is
a global config to load. If this is not the case, the user isn't able
to write to the global config without creating the backend and adding
it themselves, which is inconvenient and overly complex.

Unconditionally create and add a backend for the global config file
regardless of whether it exists as a convenience for users.

To enable this, we allow creating backends to files that do not exist
yet, changing the semantics somewhat, and making some tests invalid.
config: convenience function to open global/xdg

The rules for which one to open is a bit silly, so let's make it
easier for our users.
Merge pull request #1552 from carlosmn/config-helpers

Config helpers for global/xdg config files