Move `fetch_ref` from MergeRequestDiff#ensure_commit_shas to MergeRequest

MergeRequest#create_merge_request_diff and MergeRequest#reload_diff are
the only places where we generate a new MR diff so that's where we
should fetch the ref.

This also ensures that the ref is not fetched when we call
merge_request.merge_request_diffs.create in
Github::Import#fetch_pull_requests.
Signed-off-by: Rémy Coutable <remy@rymai.me>

app/models/merge_request.rb

@@ -415,6 +415,8 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def create_merge_request_diff
+    fetch_ref
+
     # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37435
     Gitlab::GitalyClient.allow_n_plus_1_calls do
       merge_request_diffs.create
@@ -462,6 +464,7 @@ class MergeRequest < ActiveRecord::Base
     return unless open?
 
     old_diff_refs = self.diff_refs
+
     create_merge_request_diff
     MergeRequests::MergeRequestDiffCacheService.new.execute(self)
     new_diff_refs = self.diff_refs

app/models/merge_request_diff.rb

@@ -55,8 +55,6 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def ensure_commit_shas
-    merge_request.fetch_ref
-
     self.start_commit_sha ||= merge_request.target_branch_sha
     self.head_commit_sha  ||= merge_request.source_branch_sha
     self.base_commit_sha  ||= find_base_sha