Fix submodule parsing (#32571)

Fix #32568, parse `.gitmodules` correctly

---------

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

modules/git/commit_submodule.go

@@ -0,0 +1,47 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+// GetSubModules get all the submodules of current revision git tree
+func (c *Commit) GetSubModules() (*ObjectCache[*SubModule], error) {
+	if c.submoduleCache != nil {
+		return c.submoduleCache, nil
+	}
+
+	entry, err := c.GetTreeEntryByPath(".gitmodules")
+	if err != nil {
+		if _, ok := err.(ErrNotExist); ok {
+			return nil, nil
+		}
+		return nil, err
+	}
+
+	rd, err := entry.Blob().DataAsync()
+	if err != nil {
+		return nil, err
+	}
+	defer rd.Close()
+
+	// at the moment we do not strictly limit the size of the .gitmodules file because some users would have huge .gitmodules files (>1MB)
+	c.submoduleCache, err = configParseSubModules(rd)
+	if err != nil {
+		return nil, err
+	}
+	return c.submoduleCache, nil
+}
+
+// GetSubModule get the submodule according entry name
+func (c *Commit) GetSubModule(entryName string) (*SubModule, error) {
+	modules, err := c.GetSubModules()
+	if err != nil {
+		return nil, err
+	}
+
+	if modules != nil {
+		if module, has := modules.Get(entryName); has {
+			return module, nil
+		}
+	}
+	return nil, nil
+}