When debugging an mmdebstrap build, I occasionally want an interactive aptitude GUI.
Using $MMDEBSTRAP_APT_CONFIG, I can do this without bothering to install aptitude inside the built environment.

This works with minor issues:

    mmdebstrap forky /dev/null --customize-hook='APT_CONFIG=$MMDEBSTRAP_APT_CONFIG aptitude'

This works with no issues:

    mmdebstrap forky /dev/null --customize-hook='APT_CONFIG=$MMDEBSTRAP_APT_CONFIG HOME=$1/root aptitude -oDir::Aptitude::State=var/lib/aptitude' --essential-hook='mkdir -p $1/var/lib/aptitude'

    or

    mmdebstrap forky /dev/null --customize-hook='APT_CONFIG=$MMDEBSTRAP_APT_CONFIG HOME=$1/root aptitude -oDir::Aptitude::State=var/lib/aptitude' --include=aptitude

Should mmdebstrap set Dir::Aptitude::State "var/lib/aptitude" in MMDEBSTRAP_APT_CONFIG?

mmdebstrap already sets Dir in MMDEBSTRAP_APT_CONFIG.
Should aptitude default Dir::Aptitude::State be relative to Dir (not absolute) by default?
i.e. remove leading / from the path here:
https://sources.debian.org/src/aptitude/0.8.13-8.1/configure.ac?hl=636#L636

You can see an example with relative paths here:

https://sources.debian.org/src/apt/3.3.0/test/libapt/configuration_test.cc?hl=95#L90-L95