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.

Fixes #1455

There were a number of functions assigning their return value to
`error` without much explanation.  I added in some rudimentary
error checking to help flesh out the example.

Also, I reformatted all of the comments down to 80 cols (and in
some cases, slightly updated the wording).

This makes the diff functions that take callbacks both take
the payload parameter after the callback function pointers and
pass the payload as the last argument to the callback function
instead of the first.  This should make them consistent with
other callbacks across the API.