Fix the CI build for pre-patched sources

.gitlab-ci.yml

 include: "https://git.autistici.org/ai3/build-deb/raw/master/ci-common.yml"
 
+# debian/patches patches are kept applied in the tree, no need to
+# double-apply them when building the source package.
+# Thus remove the patches altogether and build an original tarball out of HEAD.
+build_pkgsrc:stretch:
+  script:
+    - rm -rf debian/patches
+    - git archive --prefix orig/ --output ../$(dpkg-parsechangelog -SSource)_$(dpkg-parsechangelog -SVersion | sed -e  's/-[^-]*$//' -e 's/^[0-9]*://').orig.tar.gz HEAD
+    # nicer version with git-buildpackage >= 0.9.8
+    #- gbp export-orig  --upstream-tag=HEAD
+    - build-dsc