Mercurial convert extension not able to pull from remote Git repository

Question

I'm trying to do a 'hg convert', to pull from a Git repo into an Hg one.

Now, I don't have Git installed on my Windows 7 machine; is that necessary? I'm using the TortoiseHG commandline, and I have activated the convert extension ('hg help convert' works fine).

Here's an example of the command I'm trying to use:

hg convert -s git -d hg https://github.com/mysticbob/glm.git gittest

That's a public repo on Github, so I should be able to convert from it. And the address is what Github says one should use for getting. What I get is the following message:

initializing destination gittest repository
https://github.com/mysticbob/glm.git does not look like a Git repository

Any ideas?