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
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL