Dpkg::Source::Patch: Sort files inside new diff lexicographically

The files inside the diff were being sorted correctly for existing
patches, but not for new ones.

Closes: #689193

debian/changelog

@@ -108,6 +108,8 @@ dpkg (1.17.0) UNRELEASED; urgency=low
     Closes: #681371
   * Document that «dpkg-parsechangelog -l-» can be used to read from stdin.
   * Support parsing compressed changelog files transparently. Closes: #684196
+  * Sort files inside new diffs generated by dpkg-source lexicographically.
+    Closes: #689193
 
   [ Updated programs translations ]
   * Fix typo in Spanish translation of update-alternatives.

scripts/Dpkg/Source/Patch.pm

@@ -257,6 +257,8 @@ sub add_diff_directory {
         }
         @diff_files = sort { $patchorder{$a->[0]} <=> $patchorder{$b->[0]} }
                       @diff_files;
+    } else {
+        @diff_files = sort { $a->[0] cmp $b->[0] } @diff_files;
     }
 
     foreach my $diff_file (@diff_files) {