Prefer 'sudo' group when it exists; alow either git or git-core to satisfy us

Between 10.04 and 12.04, a number of changes happened that are biting us here.
First is that git replaced git-core, but there is no git package in lucid (by
default, at any rate).  Second, the general administrative group changed in
name from admin to sudo.  While sudoers still handles users in the admin group
to continue exercising that authority, by default the admin group does not
exist on new precise installs.

These changes are just packaging changes.