Add tea clone (#411)

Adds a new subcommand to clone repos:
```
tea clone --login try --depth 1 norwin/test
tea clone gitea/tea
tea clone noerw/tea           # will set up `master` to track `upstream` remote
tea clone try.gitea.io/noerw/test # will automatically set --login
```

This is just a replacement for `git clone` with small benefits:
- [x] does not depend on `git`, as tea ships with go-git
- [x] spares you typing of URLs and autoselects https/ssh based on your login config
- [x] forked repos: set up origin + upstream remote

Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Reviewed-on: https://gitea.com/gitea/tea/pulls/411
Reviewed-by: Andrew Thornton <art27@cantab.net>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

modules/task/repo_clone.go

@@ -0,0 +1,93 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package task
+
+import (
+	"fmt"
+	"net/url"
+
+	"code.gitea.io/sdk/gitea"
+	"code.gitea.io/tea/modules/config"
+	local_git "code.gitea.io/tea/modules/git"
+
+	"github.com/go-git/go-git/v5"
+	git_config "github.com/go-git/go-git/v5/config"
+	"github.com/go-git/go-git/v5/plumbing"
+)
+
+// RepoClone creates a local git clone in the given path, and sets up upstream remote
+// for fork repos, for good usability with tea.
+func RepoClone(
+	path string,
+	login *config.Login,
+	repoOwner, repoName string,
+	callback func(string) (string, error),
+	depth int,
+) (*local_git.TeaRepo, error) {
+
+	repoMeta, _, err := login.Client().GetRepo(repoOwner, repoName)
+	if err != nil {
+		return nil, err
+	}
+
+	originURL, err := cloneURL(repoMeta, login)
+	if err != nil {
+		return nil, err
+	}
+
+	auth, err := local_git.GetAuthForURL(originURL, login.Token, login.SSHKey, callback)
+	if err != nil {
+		return nil, err
+	}
+
+	// default path behaviour as native git
+	if path == "" {
+		path = repoName
+	}
+
+	repo, err := git.PlainClone(path, false, &git.CloneOptions{
+		URL:             originURL.String(),
+		Auth:            auth,
+		Depth:           depth,
+		InsecureSkipTLS: login.Insecure,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	// set up upstream remote for forks
+	if repoMeta.Fork && repoMeta.Parent != nil {
+		upstreamURL, err := cloneURL(repoMeta.Parent, login)
+		if err != nil {
+			return nil, err
+		}
+		upstreamBranch := repoMeta.Parent.DefaultBranch
+		repo.CreateRemote(&git_config.RemoteConfig{
+			Name: "upstream",
+			URLs: []string{upstreamURL.String()},
+		})
+		repoConf, err := repo.Config()
+		if err != nil {
+			return nil, err
+		}
+		if b, ok := repoConf.Branches[upstreamBranch]; ok {
+			b.Remote = "upstream"
+			b.Merge = plumbing.ReferenceName(fmt.Sprintf("refs/heads/%s", upstreamBranch))
+		}
+		if err = repo.SetConfig(repoConf); err != nil {
+			return nil, err
+		}
+	}
+
+	return &local_git.TeaRepo{Repository: repo}, nil
+}
+
+func cloneURL(repo *gitea.Repository, login *config.Login) (*url.URL, error) {
+	urlStr := repo.CloneURL
+	if login.SSHKey != "" {
+		urlStr = repo.SSHURL
+	}
+	return local_git.ParseURL(urlStr)
+}