Before a distupgrade, check that etckeeper's AVOID_COMMIT_BEFORE_INSTALL is turned off

This is something that I as the system administrator should really remember to do, but since ubuntu distribution upgrades only happen every six months, it's very easy to forget.

If this is turned on, the upgrade is guaranteed to fail midway through, resulting in a painful recovery for the user from the command line.

I'm not entirely sure that this is isn't an etckeeper bug though: if etckeeper was able to detect that it's in the middle of a distupgrade it could ignore the setting itself. But I'm not sure if that's possible or not.