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.
7 jobs for branch/default in 2 minutes and 30 seconds
Status Job ID Name Coverage
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tests-py2

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  Compat
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install

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passed #84655
tests-py2-hg-default

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tests-py2-hg-stable

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tests-py3-hg-default

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tests-py3-hg-stable

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