Fix loadOneBranch panic (go-gitea#29938)

Try to fix go-gitea#29936

Far from ideal, but still better than panic.

Author: wxiaoguang

modules/git/repo.go

@@ -283,7 +283,7 @@ type DivergeObject struct {
 // GetDivergingCommits returns the number of commits a targetBranch is ahead or behind a baseBranch
 func GetDivergingCommits(ctx context.Context, repoPath, baseBranch, targetBranch string) (do DivergeObject, err error) {
 	cmd := NewCommand(ctx, "rev-list", "--count", "--left-right").
-		AddDynamicArguments(baseBranch + "..." + targetBranch)
+		AddDynamicArguments(baseBranch + "..." + targetBranch).AddArguments("--")
 	stdout, _, err := cmd.RunStdString(&RunOpts{Dir: repoPath})
 	if err != nil {
 		return do, err

services/repository/branch.go

@@ -158,10 +158,7 @@ func loadOneBranch(ctx context.Context, repo *repo_model.Repository, dbBranch *g
 	p := protectedBranches.GetFirstMatched(branchName)
 	isProtected := p != nil
 
-	divergence := &git.DivergeObject{
-		Ahead:  -1,
-		Behind: -1,
-	}
+	var divergence *git.DivergeObject
 
 	// it's not default branch
 	if repo.DefaultBranch != dbBranch.Name && !dbBranch.IsDeleted {
@@ -180,6 +177,11 @@ func loadOneBranch(ctx context.Context, repo *repo_model.Repository, dbBranch *g
 		}
 	}
 
+	if divergence == nil {
+		// tolerate the error that we cannot get divergence
+		divergence = &git.DivergeObject{Ahead: -1, Behind: -1}
+	}
+
 	pr, err := issues_model.GetLatestPullRequestByHeadInfo(ctx, repo.ID, branchName)
 	if err != nil {
 		return nil, fmt.Errorf("GetLatestPullRequestByHeadInfo: %v", err)