Graft Tests
Add GraftedCommit object
Tests for repos with grafts
TODO: Object Stores to return GraftedCommits
getitem returns GraftedCommit

ObjectStores now have a grafts dict property

TODO: Repo is responsible for maintaining the grafts dictionary of
the objectstore.
TOOD: parse info/grafts in repo
repos parse info/grafts
object_store does not read info/grafts
Fix logic
Add existing_parent_graft
fix fake parent logic
Refactor parse_info_grafts to toplevel
parse_info_grafts -> parse_graftpoints
WIP: add grafts
WIP: tests for add/write grafts
fix typos
No self in top-level methods
serialize graftpoints
remove_grafts()
Test equality with sha/serialize