I think it would be good if sort by Name users in filters for "Assign To" and context menu "Assign To"
below some changes which I made for this

********************** Issues controller **********************

@assignables = @issue.assignable_users.sort

== changed to ==

@assignables = @issue.assignable_users.sort {|a,b| a.name <=> b.name}

********************** Query.rb **********************

user_values += project.users.sort.collect{|s| [s.name, s.id.to_s] }

== changed to ==

user_values += project.users.sort {|a,b| a.name <=> b.name}.collect{|s| [s.name, s.id.to_s] }