deb: Always provide a "changes" file

lintian in Ubuntu 14.04 now errors when a file
/usr/share/NAME/changelog.Debian.gz doesn't exist.

In the spirit of respecting lintian's meaningful errors, this commit
fixes that error.

The new behavior is that a changelog file is *always* provided. If the
--deb-changelog flag is not given, then fpm will produce a default one
based on the package being built in hope that it satisfies lintian.

With this commit, the lintian rspec test passes on Ubuntu 14.04.

Fixes #784

lib/fpm/package/deb.rb

@@ -378,13 +378,24 @@ def output(output_path)
           "Unknown compression type '#{self.attributes[:deb_compression]}'"
     end
 
-    if attributes[:deb_changelog]
-      dest_changelog = File.join(staging_path, "usr/share/doc/#{name}/changelog.Debian")
-      FileUtils.mkdir_p(File.dirname(dest_changelog))
-      FileUtils.cp attributes[:deb_changelog], dest_changelog
-      File.chmod(0644, dest_changelog)
-      safesystem("gzip", dest_changelog)
-    end
+    # Write the changelog file
+    dest_changelog = File.join(staging_path, "usr/share/doc/#{name}/changelog.Debian.gz")
+    FileUtils.mkdir_p(File.dirname(dest_changelog))
+    File.new(dest_changelog, "wb", 0644).tap do |changelog|
+      Zlib::GzipWriter.new(changelog, Zlib::BEST_COMPRESSION).tap do |changelog_gz|
+        if attributes[:deb_changelog]
+          @logger.info("Writing user-specified changelog", :source => attributes[:deb_changelog])
+          File.new(attributes[:deb_changelog]).tap do |fd|
+            chunk = nil
+            # Ruby 1.8.7 doesn't have IO#copy_stream
+            changelog_gz.write(chunk) while chunk = fd.read(16384)
+          end.close
+        else
+          @logger.info("Creating boilerplate changelog file")
+          changelog_gz.write(template("deb/changelog.erb").result(binding))
+        end
+      end.close
+    end # No need to close, GzipWriter#close will close it.
 
     attributes.fetch(:deb_init_list, []).each do |init|
       name = File.basename(init, ".init")

templates/deb/changelog.erb

@@ -0,0 +1,5 @@
+<%= name %> (<%= "#{epoch}:" if epoch %><%= version %><%= "-" + iteration.to_s if iteration %>) whatever; urgency=medium
+
+  * Package created with FPM.
+
+ -- <%= maintainer %>[two spaces]  <%= Time.now.strftime("%a, %d %b %Y %T %z") %>