The Debian packaging of git-annex 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 1.2.3, you could use:

    % git clone https://git.dgit.debian.org/git-annex
    % cd git-annex
    % git log --oneline 1.2.3..debian/1.2.3-1 -- . ':!debian'

(If you have dgit, use `dgit clone git-annex`, rather than plain `git
clone`.)

A single combined diff, containing all the changes, follows.
--- git-annex-6.20180509.orig/Utility/Url.hs
+++ git-annex-6.20180509/Utility/Url.hs
@@ -322,7 +322,8 @@ download meterupdate url file uo =
 			_ -> show he
 #else
 		let msg = case he of
-			StatusCodeException status _ _ -> statusMessage status
+			StatusCodeException status _ _ -> 
+				B8.toString (statusMessage status)
 			_ -> show he
 #endif
 		hPutStrLn stderr $ "download failed: " ++ msg
