Handle error when fetching ref for MR with deleted source branch

If the ref doesn't exist, and the source branch is deleted, we can't get it back
easily. Previously, we ignored this error by shelling out, so replicate that
behaviour.

1 files changed, 7 insertions, 2 deletions

diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index 8c1df650567..6baff362dad 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -947,7 +947,11 @@ module Gitlab
         if start_repository == self
           yield commit(start_branch_name)
         else
-          sha = start_repository.commit(start_branch_name).sha
+          start_commit = start_repository.commit(start_branch_name)
+
+          return yield nil unless start_commit
+
+          sha = start_commit.sha
 
           if branch_commit = commit(sha)
             yield branch_commit
@@ -976,8 +980,9 @@ module Gitlab
         with_repo_branch_commit(source_repository, source_branch) do |commit|
           if commit
             write_ref(local_ref, commit.sha)
+            true
           else
-            raise Rugged::ReferenceError, 'source repository is empty'
+            false
           end
         end
       end