Fixed diff test
Re-factored git-strings type translation

Removed %strings type parser that was not actually being used.
Fixed printing of garbage collected objects
Some minor improvements to the diff api
Renamed head-orphaned-p to head-unborn-p

In 0.20.0 git-repository-head-orphaned has been renamed
git-repository-head-unborn,  we are going to follow suite.
Renamed git-ls to ls-remote

This patch also updates the API to support 0.20.0
Updated diff interface to work with 0.20.0
Config struct now uses keywords for slot names

This makes it easier to use the convert-from-foreign function to convert
the struct.
Typo in documentation
Removed unused find-oid and find-oids functions
oid now uses translate-to-foreign correctly

This changes most of the oid code to more correctly use the conversion
to and from foreign.
Added missing export of index-entries symbol