Code review comment for ~pappacena/launchpad:create-mp-refs

Just to keep the record, it turn out that filtering git fetches is not that trivial, since there is no hook for that anymore (http://git.661346.n2.nabble.com/Removal-of-post-upload-hook-td4394312.html).

We will handle private repositories the following way:

1- When opening the MP, if the source is a different private repository, the virtual ref will not be created;
2- When opening the MP, if the source is the same repository (regardless of being private or not), we will create the virtual ref;
3- When opening the MP, if the owner of the source repository is the same as the owner of the target repository, we will creat ethe virtual ref
4- When privacy or the owner of a repository changes, we need to run a job to check again the above conditions and make sure that the virtual ref is created/deleted accordingly

I'll split in some MPs just to make the review easier (this MP is already a bit too big), but I guess all of these rules should be landed at once.