Use case-insensitive matching for Git error "Not a valid object name" (#36728)

Fixes #36727

Git is lowercasing the `fatal: Not a valid object name` error message
to follow its CodingGuidelines. This change makes the string matching
case-insensitive so it works with both the current and future Git
versions.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/tree_nogogit.go

@@ -65,7 +65,7 @@ func (t *Tree) ListEntries() (Entries, error) {
 
 	stdout, _, runErr := gitcmd.NewCommand("ls-tree", "-l").AddDynamicArguments(t.ID.String()).WithDir(t.repo.Path).RunStdBytes(t.repo.Ctx)
 	if runErr != nil {
-		if strings.Contains(runErr.Error(), "fatal: Not a valid object name") || strings.Contains(runErr.Error(), "fatal: not a tree object") {
+		if gitcmd.IsStdErrorNotValidObjectName(runErr) || strings.Contains(runErr.Error(), "fatal: not a tree object") {
 			return nil, ErrNotExist{
 				ID: t.ID.String(),
 			}