Fix bug

Author: lunny

services/repository/branch.go

@@ -716,6 +716,16 @@ func GetBranchDivergingInfo(ctx reqctx.RequestContext, baseRepo *repo_model.Repo
 		return info, nil
 	}
 
+	// we need fetch the necessary commits from the head repo first if it's not the same repository
+	if baseRepo.ID != headRepo.ID {
+		if _, _, err := git.NewCommand("fetch", "--no-tags").
+			AddDynamicArguments(headRepo.RepoPath()).
+			AddDynamicArguments(headGitBranch.CommitID).
+			RunStdString(ctx, &git.RunOpts{Dir: baseRepo.RepoPath()}); err != nil {
+			return nil, err
+		}
+	}
+
 	// if the fork repo has new commits, this call will fail because they are not in the base repo
 	// exit status 128 - fatal: Invalid symmetric difference expression aaaaaaaaaaaa...bbbbbbbbbbbb
 	// so at the moment, we first check the update time, then check whether the fork branch has base's head