The GIT_CONFIG_LEVEL constants actually work well as an enum
because they are mutually exclusive, so this adds a typedef to
the enum and uses that everywhere that one of these constants are
expected, instead of the old code that typically used an unsigned
int.

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.