The thread was "Switching to Git" from early March (I only have a couple of messages from that thread archived and the earliest was the 6^th) but there was some earlier discussion (but still in March IIRC). You should be able to find the thread using the archive.

The current plan is to switch to git once 5.10.1 has been released but there seem to be many hinderances to that plan that I think will take some time to work out. (Or, perhaps it won't. They're a very clever bunch.)