Comment why forks get default_git_depth of 0 instead nil

and simplify ProjectCiCdSetting#set_default_git_depth

app/models/project_ci_cd_setting.rb

@@ -31,8 +31,6 @@ class ProjectCiCdSetting < ApplicationRecord
   private
 
   def set_default_git_depth
-    return if default_git_depth
-
-    self.default_git_depth = DEFAULT_GIT_DEPTH
+    self.default_git_depth ||= DEFAULT_GIT_DEPTH
   end
 end

app/services/projects/fork_service.rb

@@ -43,6 +43,9 @@ module Projects
         shared_runners_enabled:    @project.shared_runners_enabled,
         namespace_id:              target_namespace.id,
         fork_network:              fork_network,
+        # We need to set default_git_depth to 0 for the forked project when
+        # @project.default_git_depth is nil in order to keep the same behaviour
+        # and not get ProjectCiCdSetting::DEFAULT_GIT_DEPTH set on create
         ci_cd_settings_attributes: { default_git_depth: @project.default_git_depth || 0 },
         # We need to assign the fork network membership after the project has
         # been instantiated to avoid ActiveRecord trying to create it when