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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif