Send checkout sha for web hooks and services

lib/gitlab/git.rb

 module Gitlab
   module Git
     BLANK_SHA = '0' * 40
+
+    def self.extract_ref_name(ref)
+      ref.gsub(/\Arefs\/(tags|heads)\//, '')
+    end
   end
 end

lib/gitlab/push_data_builder.rb

 module Gitlab
   class PushDataBuilder
+    class << self
       # Produce a hash of post-receive data
       #
       # data = {
@@ -19,7 +20,7 @@ module Gitlab
       #   total_commits_count: Fixnum
       # }
       #
-    def self.build(project, user, oldrev, newrev, ref, commits = [])
+      def build(project, user, oldrev, newrev, ref, commits = [])
         # Total commits count
         commits_count = commits.size
 
@@ -31,6 +32,7 @@ module Gitlab
           before: oldrev,
           after: newrev,
           ref: ref,
+          checkout_sha: checkout_sha(project.repository, newrev, ref),
           user_id: user.id,
           user_name: user.name,
           project_id: project.id,
@@ -55,9 +57,21 @@ module Gitlab
 
       # This method provide a sample data generated with
       # existing project and commits to test web hooks
-    def self.build_sample(project, user)
+      def build_sample(project, user)
         commits = project.repository.commits(project.default_branch, nil, 3)
         build(project, user, commits.last.id, commits.first.id, "refs/heads/#{project.default_branch}", commits)
       end
+
+      def checkout_sha(repository, newrev, ref)
+        if newrev != Gitlab::Git::BLANK_SHA && ref.start_with?('refs/tags/')
+          tag_name = Gitlab::Git.extract_ref_name(ref)
+          tag = repository.find_tag(tag_name)
+          commit = repository.commit(tag.target)
+          commit.try(:sha)
+        else
+          newrev
+        end
+      end
+    end
   end
 end