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/network.go

@@ -0,0 +1,28 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+// Clone clones a repository using the active backend.
+func Clone(path, remoteURL string, auth *AuthMethod, depth int, insecure bool) (*TeaRepo, error) {
+	backend, err := currentBackend().Clone(path, remoteURL, auth, CloneOptions{Depth: depth, Insecure: insecure})
+	if err != nil {
+		return nil, err
+	}
+	return newTeaRepo(backend), nil
+}
+
+// AddRemote adds a new remote to the repository.
+func (r TeaRepo) AddRemote(name, remoteURL string) error {
+	return r.backend.AddRemote(name, remoteURL)
+}
+
+// SetBranchUpstream configures the branch's upstream remote.
+func (r TeaRepo) SetBranchUpstream(branchName, remoteName, remoteBranch string) error {
+	return r.backend.SetBranchUpstream(branchName, remoteName, remoteBranch)
+}
+
+// Fetch fetches updates from the named remote.
+func (r TeaRepo) Fetch(remoteName string, refspecs []string, auth *AuthMethod) error {
+	return r.backend.Fetch(remoteName, refspecs, auth)
+}