Add table for files in merge request diffs

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.

1 files changed, 2 insertions, 1 deletions

diff --git a/spec/models/merge_request_diff_spec.rb b/spec/models/merge_request_diff_spec.rb
index 25f7062860b..4ad4abaa572 100644
--- a/spec/models/merge_request_diff_spec.rb
+++ b/spec/models/merge_request_diff_spec.rb
@@ -37,7 +37,7 @@ describe MergeRequestDiff, models: true do
 
     context 'when the raw diffs are empty' do
       before do
-        mr_diff.update_attributes(st_diffs: '')
+        MergeRequestDiffFile.delete_all(merge_request_diff_id: mr_diff.id)
       end
 
       it 'returns an empty DiffCollection' do
@@ -48,6 +48,7 @@ describe MergeRequestDiff, models: true do
 
     context 'when the raw diffs have invalid content' do
       before do
+        MergeRequestDiffFile.delete_all(merge_request_diff_id: mr_diff.id)
         mr_diff.update_attributes(st_diffs: ["--broken-diff"])
       end