MergeRequest versions to compare with present duplicated values

Summary

MergeRequest compare with versions dropdown is not working properly

Steps to reproduce

Just create a MergeRequest and push different code on the source_branch of that MergeRequest

Expected behavior

I would expect in the compare with dropdown to show only options that will show something relevant, so it's going to depend in on the current version shown, so the compare with dropdown only shows versions that are previous to the one in the current version dropdown

Actual behavior

The compare with dropdown is showing all the versions plus a master entry per each version

Relevant logs and/or screenshots

/cc @dzaporozhets