import_debian does not record issues introduced via stable

import_debian.py records fixes that go into Debian stable branches from kernel.org stable branches. It should also record issues that are introduced into Debian branches in the same way.

For example, CVE-2020-16119 affects debian/stretch but this hasn't been noted automatically.

This could be done through something like:

1. For each Debian branch:
   • Look up the current released version of the linux source package. (Check both the main and security archives and use the higher version.)
   • Extract the upstream part of the version number, and map that to the git tag and stable branch names.
2. For each issue, and for each Debian branch:
   • If the issue was introduced on the corresponding stable branch, use git merge-base --is-ancestor to check whether it was introduced before the current released version.
   • If it was introduced, record the current version in the introduced-by field.

However, this would often result in recording issues as being introduced in a later version than they were really introduced. It would be better if the Debian tracker was changed to explicitly record when issues have been introduced.