Flowdock uses Gitaly, not Grit

Prior to this change, Flowdock used Grit to get the difference between
commits and post that to the remote service. This required direct path
access, which doesn't work with Gitaly.

Fixes gitlab-org/gitaly#1113

app/models/project_services/flowdock_service.rb

 require "flowdock-git-hook"
 
+# Flow dock depends on Grit to compute the number of commits between two given
+# commits. To make this depend on Gitaly, a monkey patch is applied
+module Flowdock
+  class Git
+    # pass down a Repository all the way down
+    def repo
+      @options[:repo]
+    end
+
+    def config
+      {}
+    end
+
+    def messages
+      Git::Builder.new(repo: repo,
+                       ref: @ref,
+                       before: @from,
+                       after: @to,
+                       commit_url: @commit_url,
+                       branch_url: @branch_url,
+                       diff_url: @diff_url,
+                       repo_url: @repo_url,
+                       repo_name: @repo_name,
+                       permanent_refs: @permanent_refs,
+                       tags: tags
+                      ).to_hashes
+    end
+
+    class Builder
+      def commits
+        @repo.commits_between(@before, @after).map do |commit|
+          {
+            url: @opts[:commit_url] ? @opts[:commit_url] % [commit.sha] : nil,
+            id: commit.sha,
+            message: commit.message,
+            author: {
+              name: commit.author_name,
+              email: commit.author_email
+            }
+          }
+        end
+      end
+    end
+  end
+end
+
 class FlowdockService < Service
   prop_accessor :token
   validates :token, presence: true, if: :activated?
@@ -34,7 +80,7 @@ class FlowdockService < Service
       data[:before],
       data[:after],
       token: token,
-      repo: project.repository.path_to_repo,
+      repo: project.repository,
       repo_url: "#{Gitlab.config.gitlab.url}/#{project.full_path}",
       commit_url: "#{Gitlab.config.gitlab.url}/#{project.full_path}/commit/%s",
       diff_url: "#{Gitlab.config.gitlab.url}/#{project.full_path}/compare/%s...%s"