Gittle.rm_all: cache index property value so our changes persist

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
Gittle: add create_orphan_branch API to emulate git checkout --orphan (WIP partial implementation)

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
Gittle.create_orphan_branch: implement optional empty_index parameter

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
Gittle: avoid deprecated dulwich Tree.entries

bleeding-edge dulwich has moved the deprcated entries property of Tree
objects to '_entries', so migrate to a supported equivalent:
Tree.items().

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
Merge pull request #27 from gmt/master

Implement Gittle.create_orphan_branch API, akin to git checkout --orphan & prerequisite bugfix; avoid deprecated Tree.entries