GitLab pre-receive: pass new vcs='hg' parameter
Related to heptapod#295: this parameter will make it possible to differentiate between calls to the `/allowed` internal API endpoint for pre-receive checks (common among supported VCSes) and for Git client SSH operation. Since it'll make the pre-receive call use the `HgAccess` class instead of the previously used `GitAccess`, we need now also to pass the corresponding Mercurial wire protocol command name for pushes.