Proposed change

In mybinder.org, add a new choice to specify a repository just after "Git repository": "Mercurial repository".

Then, the information to give would be very similar than for the case "Git repository":

• "Arbitrary hg repository URL (http://hg.example.com/repo)"
• hg branch, tag, topic, bookmark or commit SHA

Who would use this feature?

Mercurial users.

One may argue that monopoly in version control is pretty bad for versioning and open-source. Without Git challenger, versioning is not as good as it could be. For example, Mercurial community (people at Facebook) invented the command absorb which is really useful and I guess Git will get something similar someday.

Mercurial is still alive, with good corporate usage and an active community. The Gitlab fork supporting Mercurial (Heptapod, used for example by PyPy) will soon reach its 1.0 version.

(Optional): Suggest a solution

The implementation for MyBinder should be quite simple : tiny changes in the website, one variable about the kind of repo (git or hg), few diff for the hg commands.