evolve: use correct functions to set current topic and topic namespace

In some cases in evolve we need to relocate commits on disk instead of purely in-memory. Previously we were writing .hg/topic file by hand here, but it wasn't the right thing to do, and also we could sometimes write an empty string to the file. Let's use the functions in topic that are specifically made to change current topic and tns.

I don't think we can simply manipulate commit extras to copy topic and topic_namespace, because sometimes we can have unresolved merge conflicts (since we're dealing with relocating on disk), and in this case users might need to continue evolve later after resolving the situation, so I think we definitely want to write current topic and tns to the files for persistence.