TESTSFIX: t0000-config.sh may unintentionally pick up existing user config in $XDG_CONFIG_HOME/todo/config

[inkarkat]

This corner case was missed when the XDG alternative config location was introduced, likely because the variable isn't set by default on Ubuntu or Mac OS, and none of the developers had a config there. HOME is already redirected to the current working directory; as XDG_CONFIG_HOME is just a config alternative that has a fallback to $HOME/.config, simply unset it.

Fixes #347

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[inkarkat]

@karbassi I've just went over all outstanding PRs; the list is quite long already. Would you be so kind to merge the fixes and approved PRs?! Also, the last release happened almost a year ago...