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_test.go

@@ -0,0 +1,74 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+type fakeBackend struct{}
+
+type fakeRepoBackend struct {
+	workTree string
+}
+
+func (fakeBackend) Name() string { return "fake" }
+
+func (fakeBackend) Open(path string) (RepositoryBackend, error) {
+	return &fakeRepoBackend{workTree: "open:" + path}, nil
+}
+
+func (fakeBackend) Clone(path, remoteURL string, auth *AuthMethod, opts CloneOptions) (RepositoryBackend, error) {
+	return &fakeRepoBackend{workTree: fmt.Sprintf("clone:%s:%s", path, remoteURL)}, nil
+}
+
+func (r *fakeRepoBackend) WorkTree() string { return r.workTree }
+func (r *fakeRepoBackend) Config() (*Config, error) {
+	return &Config{Remotes: map[string]*RemoteConfig{}, Branches: map[string]*Branch{}}, nil
+}
+
+func (r *fakeRepoBackend) Head() (*Reference, error) {
+	return &Reference{name: NewBranchReferenceName("main"), hash: Hash("deadbeef")}, nil
+}
+func (r *fakeRepoBackend) AddRemote(name, remoteURL string) error { return nil }
+func (r *fakeRepoBackend) SetBranchUpstream(branchName, remoteName, remoteBranch string) error {
+	return nil
+}
+
+func (r *fakeRepoBackend) Fetch(remoteName string, refspecs []string, auth *AuthMethod) error {
+	return nil
+}
+
+func (r *fakeRepoBackend) CreateTrackingBranch(localBranchName, remoteBranchName, remoteName string) error {
+	return nil
+}
+func (r *fakeRepoBackend) Checkout(ref ReferenceName) error          { return nil }
+func (r *fakeRepoBackend) DeleteLocalBranch(branchName string) error { return nil }
+func (r *fakeRepoBackend) DeleteRemoteBranch(remoteName, remoteBranch string, auth *AuthMethod) error {
+	return nil
+}
+func (r *fakeRepoBackend) ListReferences(prefixes ...string) ([]*Reference, error) { return nil, nil }
+func (r *fakeRepoBackend) PushToAgitFlowPR(remoteName, head, base, topic string, pushOptions map[string]string, auth *AuthMethod) error {
+	return nil
+}
+
+func TestCanSwitchBackends(t *testing.T) {
+	setBackendForTesting(t, fakeBackend{})
+
+	repo, err := RepoFromPath("demo")
+	require.NoError(t, err)
+	require.Equal(t, "open:demo", repo.WorkTree())
+	require.Equal(t, "fake", CurrentBackendName())
+
+	cloned, err := Clone("target", "https://example.com/repo.git", nil, 1, false)
+	require.NoError(t, err)
+	require.Equal(t, "clone:target:https://example.com/repo.git", cloned.WorkTree())
+}
+
+func TestRegisteredBackendsContainsCLI(t *testing.T) {
+	require.Contains(t, RegisteredBackends(), "cli")
+}