This adds a GIT-- object type to git-annex.

GIT--manifest is the manifest

GIT--hash is a git bundle

# format of the manifest file

An ordered list of bundle keys, one per line.

# fetching

1. download manifest
2. download each listed GIT bundle object that we don't have
3. fetch from bundles in timestamp order

# pushing

1. create git bundle, hash to calculate GIT bundle object name
2. upload GIT bundle object
3. download current manifest
4. add to manifest with current time, and upload