Thanks to everyone for great recommendations! Many fine options.
Running with Fork for now: it has a great commit workflow, once I got my head around it. (Being able to commit selected fragments, even partial lines, is a killer feature).
SourceTree and Tower are also really nice, along the high-polish lines I was looking for. Mercurial support in SourceTree is a big plus; keeping it around just for that.
As long as we’re sharing recos, I want to put in a good word for https://gitup.co.