* pygit2: implement get/set/remove refs
* pygit: implement iter_refs
* pygit: implement resolve_rev
* pygit2: implement working tree checkout
* pygit2: implement reset, explicitly read/write index when used
* pygit2: checkout does not need to reset index
* pygit2: handle "-" branch shorthand in checkout
* tests: install post-checkout hook should be run via gitpython
* exp apply: use stricter exception handling for merge conflict
* pygit: fix index read/write usage
* gitpython: handle untracked file conflicts in stash apply
* fix flaky/race index issue