If the default branch is not present do not report error on stats indexing (follow-up of #15546) (#15583)

#15546 doesn't completely fix this problem because the error returned is an ObjectNotExist
error not a BranchNotExist error.

Add test for ErrObjectNotExist too

Fix #15257

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/indexer/stats/db.go

@@ -38,7 +38,7 @@ func (db *DBIndexer) Index(id int64) error {
 	// Get latest commit for default branch
 	commitID, err := gitRepo.GetBranchCommitID(repo.DefaultBranch)
 	if err != nil {
-		if git.IsErrBranchNotExist(err) {
+		if git.IsErrBranchNotExist(err) || git.IsErrNotExist((err)) {
 			log.Debug("Unable to get commit ID for defaultbranch %s in %s ... skipping this repository", repo.DefaultBranch, repo.RepoPath())
 			return nil
 		}