debian: define extend-diff-ignore for dpkg-source
This makes it possible to build the source package from a hg checkout
(and a sdist) by having dpkg-source ignore files not distributed in
sdist.
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/debian/source/options Tue Oct 15 11:39:58 2019 +0200
@@ -0,0 +1,1 @@
+extend-diff-ignore = "^(contrib/|.gitlab-ci.yml|.hg-format-source|MANIFEST|Makefile|docs/tutorial/.netlify|hgext3rd/evolve/hack/|tests/.testtimes)"