I don't know but I fear that - if done automatically - it might want to keep the most recent changes of both branches.
From now on any changes I make will go into Large Rivers.
I try to download frequently, if only so that the German text I’m translating is the most recent. The main branch folder I’m using right now is from March 27, 2021.
Large Rivers is suggested in March 2020, a little over a year ago, and really gets going in October, so we don't need to worry about what happened before that.
28 documents were last changed two years ago, so it should be possible to replace all those from the main branch whenever it has updates. A further 29 are five months ago, each of them ‟Text Improvements from Kendon”, so again more recent changes in the main branch should be easy to commit
That leaves 23 documents where Large Rivers text could be different from the main branch. Looking at the comments I’d guess several of those might not be big changes into Large Rivers, though main branch might show new translations, for example.
There are 9 where Large Rivers has recent updates: still a big job manually, but more manageable than everything all over again.
It's good you do so much that I can’t. I’m getting a practical appreciation for how this is, and always has been, a real team effort.