The Debian packaging of foo is maintained in git, using the merging
workflow described in dgit-maint-merge(7). There isn't a patch queue that
can be represented as a quilt series.
A detailed breakdown of the changes is available from their canonical
representation: git commits in the packaging repository. For example, to
see the changes made by the Debian maintainer in the first upload of
upstream version 3.0, you could use:
% git clone https://git.eyrie.org/git/debian/xfonts-jmk.git
% cd xfonts-jmk
% git log --oneline 3.0..debian/3.0-1 -- . ':!debian'
(If you have dgit, use `dgit clone foo`, rather than plain `git clone`.)
A single combined diff, containing all the changes, follows.