README: add Compatibility, improve Installing

git_handler: properly handle paths when files converge

filectx.renamed() returns a 2-tuple or None. memfilectx.__init__ expects
the copied argument to be either None or a string. Before, we were
passing a 2-tuple, leading to the memfilectx storing the wrong type.
This eventually resulted in doing a key lookup against a manifest
with a 2-tuple, which made manifest.c throw an error.

git_handler: update for bookmark changes in mercurial 3.5

tests: update tests for new git output, sigh

Added tag 0.8.1 for changeset e6489cf3fe8c

setup: mark version as 0.8.1

init: mark hg-git as compatible with Mercurial 3.4

Note that one test still fails with 3.4 -- however, it is a bug in core
Mercurial, only affects edge cases (broken symlinks) in the test, and is fixed
in upstream stable.

overlaychangectx: use overlayrevlog logic to determine parents

The old logic was broken -- it didn't work at the boundary between hg and git
commits. The logic in overlayrevlog.parents handles that correctly.

This is the last fix required for Mercurial 3.4.

overlay: adapt diff to work with lazymanifests

test-bookmark-workflow: fix for Mercurial 3.4

pull: acquire wlock before lock to fix lock-checker warnings

overlay: flake8 cleanup

overlay: remove method that apparenetly was never called

This method had a syntax error that would have caused a crash.

hg2git: flake8 cleanup

gitdirstate: flake8 cleanup

verify: flake8 cleanup

util: flake8 cleanup

gitrepo: flake8 cleanups

ssh: move imports to top of file

hgrepo: remove unused import