git: add support for working with refspecs and stashes (#4952)

* 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

setup.py

@@ -50,7 +50,7 @@ install_requires = [
     "colorama>=0.3.9",
     "configobj>=5.0.6",
     "gitpython>3",
-    "dulwich>=0.20.11",
+    "dulwich>=0.20.14",
     "setuptools>=34.0.0",
     "nanotime>=0.5.2",
     "pyasn1>=0.4.1",