Handle the unadvertised object error

What does this MR do and why?

Contributes to #427699 (closed)

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/449207+

The change is behind of the feature flag: #427699 (closed)

Problem

When user imports a repository from the BitBucket Server, we are trying to fetch missing commits. See more information: #356240 (closed).

But we discovered a problem with repositories that have submodules. The commit fetching process fails for them, because we have to fetch submodules first.

Solution

As a workaround to unblock the import process, we can skip loading of specific commits when the repository has submodules.