Thanks, @eyedear, and lovely that you want to test it!
You do not need to merge it, just checkout the branch. There are some suggestions at the beginning of this thread, I would suggest just do a fresh clone of my fork for the least amount of git work: GitHub - jandren/darktable at sigmoid_tone_mapping
As for compiling and using, please install as a dev version as it will fuck up your database otherwise. See the darktable main Readme.
And then just fire away! Note that the parameters aren’t fully orthogonal yet so you will need to adjust the contrast after adjusting the skew to maintain the contrast from before. Looking forward to your comments, especially if you find images where it struggles! Weird edge cases are my favorites 
Seriously, those are the once where you find bugs and problems!