Introduce a refs iterator

If the backend doesn't provide support for it, the matching is done in
refdb on top of a normal iterator.

Nobody should ever be using anything other than ALL at this level, so
remove the option altogether.

As part of this, git_reference_foreach_glob is now implemented in the
frontend using an iterator. Backends will later regain the ability of
doing the glob filtering in the backend.

This allows us to get a list of reference names in a loop instead of callbacks.

This clarifies the docs for git_repository_message and also adds
to the tests to explicitly check NUL termination of data when the
output buffer is smaller than the message size.  There is a minor
behavior change so that a non-NULL output buffer will always be
NUL terminated (at length zero) if an error occurs.

Expose git_reference_dwim

Config helpers for global/xdg config files

Extract this function out of the rev-parse code to be able to DWIM a
reference instead of its target.

The rules for which one to open is a bit silly, so let's make it
easier for our users.

Diff patch bug fixes