Always compare with FETCH_HEAD in downtime_check

This ensures this CI step works properly even when doing a shallow
clone.

lib/tasks/downtime_check.rake

 desc 'Checks if migrations in a branch require downtime'
 task downtime_check: :environment do
-  # First we'll want to make sure we're comparing with the right upstream
-  # repository/branch.
-  current_branch = `git rev-parse --abbrev-ref HEAD`.strip
-
-  # Either the developer ran this task directly on the master branch, or they're
-  # making changes directly on the master branch.
-  if current_branch == 'master'
-    if defined?(Gitlab::License)
-      repo = 'gitlab-ee'
-    else
-      repo = 'gitlab-ce'
-    end
-
-    `git fetch https://gitlab.com/gitlab-org/#{repo}.git --depth 1`
-
-    compare_with = 'FETCH_HEAD'
-  # The developer is working on a different branch, in this case we can just
-  # compare with the master branch.
+  if defined?(Gitlab::License)
+    repo = 'gitlab-ee'
   else
-    compare_with = 'master'
+    repo = 'gitlab-ce'
   end
 
-  Rake::Task['gitlab:db:downtime_check'].invoke(compare_with)
+  `git fetch https://gitlab.com/gitlab-org/#{repo}.git --depth 1`
+
+  Rake::Task['gitlab:db:downtime_check'].invoke('FETCH_HEAD')
 end