Send EmailsOnPush when deleting commits using force push.

See #1924.

app/mailers/emails/projects.rb

@@ -16,9 +16,10 @@ module Emails
            subject: subject("Project was moved"))
     end
 
-    def repository_push_email(project_id, recipient, author_id, branch, compare, send_from_committer_email = false, disable_diffs = false)
+    def repository_push_email(project_id, recipient, author_id, branch, compare, reverse_compare = false, send_from_committer_email = false, disable_diffs = false)
       @project = Project.find(project_id)
       @author  = User.find(author_id)
+      @reverse_compare = reverse_compare
       @compare = compare
       @commits = Commit.decorate(compare.commits)
       @diffs   = compare.diffs
@@ -32,10 +33,13 @@ module Emails
                                                     @project,
                                                     from: @commits.first,
                                                     to: @commits.last)
+        @subject << "Deleted " if @reverse_compare
         @subject << "#{@commits.length} commits: #{@commits.first.title}"
       else
         @target_url = namespace_project_commit_url(@project.namespace,
                                                    @project, @commits.first)
+
+        @subject << "Deleted 1 commit: " if @reverse_compare
         @subject << @commits.first.title
       end
 

app/views/notify/repository_push_email.html.haml

 %h3 #{@author.name} pushed to #{@branch} at #{link_to @project.name_with_namespace, namespace_project_url(@project.namespace, @project)}
 
-%h4 Commits:
+- if @reverse_compare
+  %p
+    %strong WARNING:
+    The push did not contain any new commits, but force pushed to delete the commits and changes below.
+
+%h4
+  = @reverse_compare ? "Deleted commits:" : "Commits:"
 
 %ul
   - @commits.each do |commit|

app/views/notify/repository_push_email.text.haml

 #{@author.name} pushed to #{@branch} at #{@project.name_with_namespace}
-
 \
-Commits:
+\
+- if @reverse_compare
+  WARNING: The push did not contain any new commits, but force pushed to delete the commits and changes below.
+  \
+  \
+= @reverse_compare ? "Deleted commits:" : "Commits:"
 - @commits.each do |commit|
-  #{commit.short_id} by #{commit.author_name}
+  #{commit.short_id} by #{commit.author_name} at #{commit.committed_date.strftime("%Y-%m-%dT%H:%M:%SZ")}
   #{commit.safe_message}
   \- - - - -
 \

app/workers/emails_on_push_worker.rb

@@ -15,8 +15,15 @@ class EmailsOnPushWorker
 
     compare = Gitlab::Git::Compare.new(project.repository.raw_repository, before_sha, after_sha)
 
-    # Do not send emails if git compare failed
-    return false unless compare && compare.commits.present?
+    return false if compare.same
+
+    if compare.commits.empty?
+      compare = Gitlab::Git::Compare.new(project.repository.raw_repository, after_sha, before_sha)
+
+      reverse_compare = true
+
+      return false if compare.commits.empty?
+    end
 
     recipients.split(" ").each do |recipient|
       Notify.repository_push_email(
@@ -25,6 +32,7 @@ class EmailsOnPushWorker
         author_id, 
         branch, 
         compare, 
+        reverse_compare,
         send_from_committer_email,
         disable_diffs
       ).deliver

spec/mailers/notify_spec.rb

@@ -571,7 +571,7 @@ describe Notify do
     let(:diff_path) { namespace_project_compare_path(project.namespace, project, from: commits.first, to: commits.last) }
     let(:send_from_committer_email) { false }
 
-    subject { Notify.repository_push_email(project.id, 'devs@company.name', user.id, 'master', compare, send_from_committer_email) }
+    subject { Notify.repository_push_email(project.id, 'devs@company.name', user.id, 'master', compare, false, send_from_committer_email) }
 
     it 'is sent as the author' do
       sender = subject.header[:from].addrs[0]