Compatibility with dulwich 0.9.1

dulwich 0.9.1 introduced two API changes which affect hg-git.
SSHVendor.connect_ssh was renamed to SSHVendor.run_command and
ObjectStore.add_pack returns a 3-tuple instead of a 2-tuple.

In addition to the pure compatibility fixes, GitHandler.fetch_pack will
use the new abort function ObjectStore.add_pack returns if a new enough
dulwich is being used.

Signed-off-by: James McCoy <vega.james@gmail.com>