This is another option which we should not be keeping in the remote, but
is specific to each particular operation.

Having the setting be different from calling its actions was not a great
idea and made for the sake of the wrong convenience.

Instead of that, accept either fetch options, push options or the
callbacks when dealing with the remote. The fetch options are currently
only the callbacks, but more options will be moved from setters and
getters on the remote to the options.

This does mean passing the same struct along the different functions but
the typical use-case will only call git_remote_fetch() or
git_remote_push() and so won't notice much difference.

The text progress and update_tips callbacks are already part of the
struct, which was meant to unify the callback setup, but the download
one was left out.

git_indexer_stats and friends -> git_transfer_progress*

Also made git_transfer_progress members more sanely
named.

Also removing all the *stats parameters from external
APIs that don't need them anymore.

This allows us to add capabilitites to both at the same time, keeps
them in sync and removes a lot of code.

gitno_buffer now uses a callback to fill its buffer, allowing us to
use the same interface for git and http (which uses callbacks).