Migrate MR commits and diffs to new tables

Previously, we stored these as serialised fields - `st_{commits,diffs}` - on the
`merge_request_diffs` table. These now have their own tables -
`merge_request_diff_{commits,diffs}` - with a column for each attribute of the
serialised data.

Add a background migration to go through the existing MR diffs and migrate them
to the new format. Ignore any contents that cannot be displayed. Assuming that
we have 5 million rows to migrate, and each batch of 2,500 rows can be
completed in 5 minutes, this will take about 7 days to migrate everything.

1 files changed, 1 insertions, 5 deletions

diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index ec87aee9310..d9d746ccf41 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -85,11 +85,7 @@ class MergeRequestDiff < ActiveRecord::Base
 
   def raw_diffs(options = {})
     if options[:ignore_whitespace_change]
-      @diffs_no_whitespace ||=
-        Gitlab::Git::Compare.new(
-          repository.raw_repository,
-          safe_start_commit_sha,
-          head_commit_sha).diffs(options)
+      @diffs_no_whitespace ||= compare.diffs(options)
     else
       @raw_diffs ||= {}
       @raw_diffs[options] ||= load_diffs(options)