* git: add support for dulwich Stash
* git: start adding support for custom refs
* git: add Stash and helper context managers for preserving
workspace state
* scm: cleanup refs/stash and add push_refspec
* tests: add tests for git refs and stash
* fix push_refspec
* tests: add tests for git push_refspec
* scm: add git fetch-refspecs
* add test for fetch refspecs
* scm: add test for git refs containing
* remove extraneous log msgs
* fix typing issues
* stash: only try to push if workspace is dirty
* update dulwich to 0.20.14
* scm: move dulwich into scm.backend
* move stash internals to backends
* scm: move git code into dvc/scm/git/... module
* git: use ABC for BaseGitBackend
* fix rebase issue
* git: fix refspec tests on windows
- git file:// URLs need posix style paths
* git: move get_refs_containing to backend
* scm: add callback for handling diverged refs during fetch