Fix submodule parsing (#32571)

Fix #32568, parse `.gitmodules` correctly

---------

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

modules/git/config_submodule.go

@@ -0,0 +1,75 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// SubModule is a reference on git repository
+type SubModule struct {
+	Path   string
+	URL    string
+	Branch string // this field is newly added but not really used
+}
+
+// configParseSubModules this is not a complete parse for gitmodules file, it only
+// parses the url and path of submodules. At the moment it only parses well-formed gitmodules files.
+// In the future, there should be a complete implementation of https://git-scm.com/docs/git-config#_syntax
+func configParseSubModules(r io.Reader) (*ObjectCache[*SubModule], error) {
+	var subModule *SubModule
+	subModules := newObjectCache[*SubModule]()
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+
+		// Skip empty lines and comments
+		if line == "" || strings.HasPrefix(line, "#") || strings.HasPrefix(line, ";") {
+			continue
+		}
+
+		// Section header [section]
+		if strings.HasPrefix(line, "[") && strings.HasSuffix(line, "]") {
+			if subModule != nil {
+				subModules.Set(subModule.Path, subModule)
+			}
+			if strings.HasPrefix(line, "[submodule") {
+				subModule = &SubModule{}
+			} else {
+				subModule = nil
+			}
+			continue
+		}
+
+		if subModule == nil {
+			continue
+		}
+
+		parts := strings.SplitN(line, "=", 2)
+		if len(parts) != 2 {
+			continue
+		}
+		key := strings.TrimSpace(parts[0])
+		value := strings.TrimSpace(parts[1])
+		switch key {
+		case "path":
+			subModule.Path = value
+		case "url":
+			subModule.URL = value
+		case "branch":
+			subModule.Branch = value
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("error reading file: %w", err)
+	}
+	if subModule != nil {
+		subModules.Set(subModule.Path, subModule)
+	}
+	return subModules, nil
+}