Use git command instead of go git (#1005)

Remove go git library because it doesn't support sha256 repository but have an interface so that we could have other backend for the future.

Reviewed-on: https://gitea.com/gitea/tea/pulls/1005
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

modules/git/backend.go

@@ -0,0 +1,103 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+var backendRegistry = struct {
+	sync.RWMutex
+	current  string
+	backends map[string]Backend
+}{
+	backends: make(map[string]Backend),
+}
+
+func init() {
+	mustRegisterBackend(cliBackend{})
+	mustUseBackend("cli")
+}
+
+// RegisterBackend makes a git backend available for later switching.
+func RegisterBackend(backend Backend) error {
+	if backend == nil {
+		return fmt.Errorf("git backend is nil")
+	}
+	name := backend.Name()
+	if name == "" {
+		return fmt.Errorf("git backend name is empty")
+	}
+
+	backendRegistry.Lock()
+	defer backendRegistry.Unlock()
+	backendRegistry.backends[name] = backend
+	if backendRegistry.current == "" {
+		backendRegistry.current = name
+	}
+	return nil
+}
+
+func mustRegisterBackend(backend Backend) {
+	if err := RegisterBackend(backend); err != nil {
+		panic(err)
+	}
+}
+
+// UseBackend switches the active git backend implementation.
+func UseBackend(name string) error {
+	backendRegistry.Lock()
+	defer backendRegistry.Unlock()
+	if _, ok := backendRegistry.backends[name]; !ok {
+		return fmt.Errorf("git backend %q is not registered", name)
+	}
+	backendRegistry.current = name
+	return nil
+}
+
+func mustUseBackend(name string) {
+	if err := UseBackend(name); err != nil {
+		panic(err)
+	}
+}
+
+// CurrentBackendName returns the active backend name.
+func CurrentBackendName() string {
+	backendRegistry.RLock()
+	defer backendRegistry.RUnlock()
+	return backendRegistry.current
+}
+
+// RegisteredBackends returns all registered backend names.
+func RegisteredBackends() []string {
+	backendRegistry.RLock()
+	defer backendRegistry.RUnlock()
+	out := make([]string, 0, len(backendRegistry.backends))
+	for name := range backendRegistry.backends {
+		out = append(out, name)
+	}
+	sort.Strings(out)
+	return out
+}
+
+func currentBackend() Backend {
+	backendRegistry.RLock()
+	defer backendRegistry.RUnlock()
+	return backendRegistry.backends[backendRegistry.current]
+}
+
+func setBackendForTesting(t testingT, backend Backend) {
+	t.Helper()
+	prev := CurrentBackendName()
+	mustRegisterBackend(backend)
+	mustUseBackend(backend.Name())
+	t.Cleanup(func() { mustUseBackend(prev) })
+}
+
+type testingT interface {
+	Cleanup(func())
+	Helper()
+}