Remove extra git describe on Rake shell install.

I'm sure there is a good reason for this, but I'm just too curious to know why =)

IIUC:

- `man gitrevisions` says that refnames are already searched under `refs/heads/<name>` and then `refs/remotes/<name>` in that order by default. So saying `git checkout branch` will find `remote/origin/branch` already if `heads/branch` does not exist.

- no other type of revision name is stored under `refs/heads/remote`

cc @crohr you may have written that line, sorry if not.

Actually , what I really wanted to do is to remove the `describe` entirely: if I give a ref from the command line, it is simpler to use it directly instead of `describe ref`. People should never install the latest stable and always give the version that corresponds to their gitlab instance.