Upstream introduced the new `commits_check`, for now only
about checking signatures, and more specifically, checking
in case of a signature by GitLab (web change) that the current
user matches the commit author.

This puts the `new_commits` method in the systematic call path,
finally calling the `ListCommits` gRPC method with something like
`[head1_sha, head2_sha, '--not', '--all']`. While this is useful
with Git, as a way to inspect new commits before they actually bear
references, it just breaks every push to Mercurial native repositories.

Also, the new check accesses the commits themselves, which do
not exist in our `HgAccess` tests. We preferred to solve this
problem without any change in main code (was tempting, because
Heptapod does not support signatures yet, but would have been
dangerous as it would have risked missing new, more relevant checks).