services: in loadOneBranch, return if CountDivergingCommits fail

If we can't count the number of diverging commits for one reason or
another (such as the branch being in the database, but missing from
disk), rather than logging an error and continuing into a crash (because
`divergence` will be nil), return an error instead.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

services/repository/branch.go

@@ -136,7 +136,7 @@ func loadOneBranch(ctx context.Context, repo *repo_model.Repository, dbBranch *g
 		var err error
 		divergence, err = files_service.CountDivergingCommits(ctx, repo, git.BranchPrefix+branchName)
 		if err != nil {
-			log.Error("CountDivergingCommits: %v", err)
+			return nil, fmt.Errorf("CountDivergingCommits: %v", err)
 		}
 	}