bug#72174: guix download does not support Mercurial

[PIERRE AUGIER]

https://foss.heptapod.net/fluiddyn/fluidsim is a Mercurial repository.

```
guix download --commit=62c4dbe4f786 https://foss.heptapod.net/fluiddyn/fluidsim
```

fails with:

```
SWH: revision "62c4dbe4f786" originating from 
https://foss.heptapod.net/fluiddyn/fluidsim could not be found
guix download: warning: revision 62c4dbe4f786 of 
https://foss.heptapod.net/fluiddyn/fluidsim could not be fetched from Software 
Heritage
guix download: error: Git error: unexpected http status code: 404
```

I studied `guix download -h` which shows that `guix download` only supports Git 
repositories. Since there is `hg-download`, it would make sense that the 
command `guix download` support Mercurial.

--
Pierre Augier - CR CNRS                 http://www.legi.grenoble-inp.fr
LEGI (UMR 5519) Laboratoire des Ecoulements Geophysiques et Industriels
BP53, 38041 Grenoble Cedex, France                tel:+33.4.56.52.86.16