This adds an ID-less table containing one row per file, per merge request
diff. It has a column for each attribute on Gitlab::Git::Diff that is serialised
currently, with the advantage that we can easily query the attributes of this
new table.
It does not migrate existing data, so we have fallback code when the legacy
st_diffs column is present instead. For a merge request diff to be valid, it
should have at most one of:
* Rows in this new table, with the correct merge_request_diff_id.
* A non-NULL st_diffs column.
It may have neither, if the diff is empty.
Fixed all related specs and also changed the logic to handle edge cases. This includes exporting and exporting of group labels, which will get associated with the new group (if any) or they will become normal project labels otherwise.
Found other issues to do with not being able to import all labels at once in the beginning of the JSON - code was much simpler when we import all labels and milestones associated to a project first, then the associations will find the already created labels instead of creating them from the associations themselves.
Sean McGivern