native-migration: pre-check Gitaly and HGitaly only for relevant storages
The following discussion from !278 (merged) should be addressed:
Last remark (non-blocking): checking all storages is a bit heavy-handed actually:
- the rake task is currently meant for one project only (hence just one storage)
- we may have in the future storages with Gitaly only or with HGitaly only, as this makes sense for instances having far more projects of one VCS type than the other, but we currently don't have obvious means to know about that, at least from the rake task.
In the short term, checking only for the storage related to the given project would be enough, but we will probably have a version of the rake task to run for all projects, making these methods to check all storages useful again…