Remove oldrev and newrev nil-checks that are now unnecessary

ee/lib/ee/gitlab/checks/push_rules/branch_check.rb

@@ -9,7 +9,7 @@ module EE
           LOG_MESSAGE = "Checking if branch follows the naming patterns defined by the project...".freeze
 
           def validate!
-            return unless newrev && oldrev && push_rule
+            return unless push_rule
 
             logger.log_timed(LOG_MESSAGE) do
               unless branch_name_allowed_by_push_rule?

ee/lib/ee/gitlab/checks/push_rules/commit_check.rb

@@ -13,7 +13,7 @@ module EE
           LOG_MESSAGE = "Checking if commits follow defined push rules...".freeze
 
           def validate!
-            return unless newrev && oldrev && push_rule
+            return unless push_rule
 
             commit_validation = push_rule.commit_validation?
             # if newrev is blank, the branch was deleted

ee/lib/ee/gitlab/checks/push_rules/tag_check.rb

@@ -6,7 +6,7 @@ module EE
       module PushRules
         class TagCheck < ::Gitlab::Checks::BaseChecker
           def validate!
-            return unless newrev && oldrev && push_rule
+            return unless push_rule
 
             logger.log_timed("Checking if you are allowed to delete a tag...") do
               if tag_deletion_denied_by_push_rule?

lib/gitlab/checks/base_checker.rb

@@ -19,12 +19,16 @@ module Gitlab
 
       private
 
+      def creation?
+        Gitlab::Git.blank_ref?(oldrev)
+      end
+
       def deletion?
         Gitlab::Git.blank_ref?(newrev)
       end
 
       def update?
-        !Gitlab::Git.blank_ref?(oldrev) && !deletion?
+        !creation? && !deletion?
       end
 
       def updated_from_web?

lib/gitlab/checks/diff_check.rb

@@ -12,7 +12,7 @@ module Gitlab
       }.freeze
 
       def validate!
-        return if deletion? || newrev.nil?
+        return if deletion?
         return unless should_run_diff_validations?
         return if commits.empty?