tests: add test for issue5678 In addition to a test case for the direct problem described in the bug report, this also adds a test case showing how obsmarkers can also get lost when not using generaldelta. Differential Revision: https://phab.mercurial-scm.org/D714

merge: move cwd-missing detection to helper functions This will exist in two places with defered writes, so we want to avoid duplication. Differential Revision: https://phab.mercurial-scm.org/D626

doctest: enable tests by default on Python 3 Still several tests fail mostly because of the string issues, sigh. I'll fix them one by one.

doctest: normalize b'', u'' and exception output on Python 3 The idea is described in the following page. https://dirkjan.ochtman.nl/writing/2014/07/06/single-source-python-23-doctests.html # no-check-commit

doctest: coerce dict.keys() to list Otherwise it would be printed as odict_keys([...]) on Python 3.

doctest: upgrade old-style "except" clause

doctest: use print_function and convert bytes to unicode where needed

doctest: do not embed non-ascii characters in docstring Since the outer docstring is parsed as a unicode on Python 3, we have to either double-escape or construct non-ascii string from ascii string.