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.