[PATCH 5 of 5] cleanup: replace False identity testing with an explicit token object

[Augie Fackler]

> On May 11, 2016, at 09:34, Pierre-Yves David <pierre-yves.david at ens-lyon.org> wrote:
> 
>> (I’ll probably still take this patch, but want to continue this discussion.)
> 
> That patch is not related to these dirstate.write(None) call in localrepo I'm not sure why this discussion would delay them.
> (discussion in itself it useful, but unrelated)

It's related in my mind, because I was having a lot of trouble understanding why we're using a sentinel object instead of just requiring callers to always pass a transaction. Now that I understand, this patch is a strict improvement (so I'll take it), but there's still some work to be done to make this a clean API (which I'll probably do sometime this week.)

Thanks for bearing with me.