Bump debhelper from old 11 to 12.

Fixes lintian: package-uses-old-debhelper-compat-version
See https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html for more details.

debian/changelog

@@ -8,6 +8,9 @@ dulwich (0.19.12-1) UNRELEASED; urgency=medium
   [ Ondřej Nový ]
   * Use debhelper-compat instead of debian/compat.
 
+  [ Jelmer Vernooij ]
+  * Bump debhelper from old 11 to 12.
+
  -- Jelmer Vernooij <jelmer@debian.org>  Tue, 13 Aug 2019 19:44:44 +0000
 
 dulwich (0.19.11-2) unstable; urgency=medium

debian/control

@@ -4,7 +4,7 @@ Priority: optional
 Maintainer: Debian Python Modules Team <python-modules-team@lists.alioth.debian.org>
 Uploaders: Jelmer Vernooij <jelmer@debian.org>
 Homepage: https://www.dulwich.io/
-Build-Depends: debhelper-compat (= 11),
+Build-Depends: debhelper-compat (= 12),
                dh-python,
                git (>= 1:1.7.0.4-2) [!alpha !ppc64 !sparc64 !x32 !ia64],
                pypy-dev [alpha amd64 arm64 armel armhf i386 mips powerpc ppc64el s390x],