checks: Fix mismatch in `#new_commits()` signature

What does this MR do?

In commit 0a90028f (checks: Speed up retrieving commits via quarantine directory, 2021-07-29), the Gitlab::Git::Repository class got a new optional parameter allow_quarnatine for #new_commits(). The commit forgot to amend the Repository model though to accept the same parameter, which as a result leads to a runtime error when the access checks try to load new commits.

Fix this issue by adding the missing parameter to the Repository model. This commit also changes the changes access spec to call the original function such that the bug would've been caught without the fix.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)