debian: Require patch >= 2.7 to get git-style diff support

This guarantees that we get a reproducible behavior, as previous patch
versions did not support git-style diffs.

debian/changelog

@@ -1,5 +1,9 @@
 dpkg (1.17.21) UNRELEASED; urgency=low
 
+  [ Guillem Jover ]
+  * Get consistent git-style diff support, by adding (>= 2.7) to the dpkg-dev
+    patch Depends, and a Breaks patch (<< 2.7) to libdpkg-perl.
+
   [ Updated programs translations ]
   * Catalan (Guillem Jover).
   * German (Sven Joachim) (Closes: #766311).

debian/control

@@ -71,7 +71,7 @@ Priority: optional
 Architecture: all
 Multi-Arch: foreign
 Depends: libdpkg-perl (= ${source:Version}), bzip2, xz-utils,
- patch, make, binutils, base-files (>= 5.0.0), ${misc:Depends}
+ patch (>= 2.7), make, binutils, base-files (>= 5.0.0), ${misc:Depends}
 Recommends: gcc | c-compiler, build-essential, fakeroot,
  gnupg | gnupg2, gpgv | gpgv2, libalgorithm-merge-perl
 Suggests: debian-keyring
@@ -93,7 +93,7 @@ Depends: dpkg (>= 1.15.8), perl, libtimedate-perl, ${misc:Depends}
 Recommends: libfile-fcntllock-perl, bzip2, xz-utils
 Suggests: debian-keyring, gnupg | gnupg2, gpgv | gpgv2,
  gcc | c-compiler, binutils, patch
-Breaks: dpkg-dev (<< 1.15.6)
+Breaks: dpkg-dev (<< 1.15.6), patch (<< 2.7)
 Replaces: dpkg (<< 1.15.8), dpkg-dev (<< 1.15.6)
 Description: Dpkg perl modules
  This package provides the perl modules used by the scripts