> ...this is solved by the named branches in Mercurial... I'm not super-familiar with Mercurial (and only somewhat-familiar with git), but couldn't you get the same effect as named branches by just not deleting fully merged branches in git?

> ... how can you tell which one of the parents is the previous commit in the master ...

You can use --first-parent [1] to disambiguate that. In a nutshell, the master branch in your example would be the first parent, letting you disambiguate (and there's support for this in a lot of other git tools!)

[1]: https://git-blame.blogspot.ca/2012/03/fun-with-first-parent....