Add support to migrate from gogs (#14342)

Add support to migrate gogs:

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  *  wiki


Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>

vendor/github.com/gogs/go-gogs-client/repo_commit.go

@@ -0,0 +1,53 @@
+// Copyright 2018 The Gogs 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 gogs
+
+import (
+	"fmt"
+	"net/http"
+)
+
+// CommitMeta contains meta information of a commit in terms of API.
+type CommitMeta struct {
+	URL string `json:"url"`
+	SHA string `json:"sha"`
+}
+
+// CommitUser contains information of a user in the context of a commit.
+type CommitUser struct {
+	Name  string `json:"name"`
+	Email string `json:"email"`
+	Date  string `json:"date"`
+}
+
+// RepoCommit contains information of a commit in the context of a repository.
+type RepoCommit struct {
+	URL       string      `json:"url"`
+	Author    *CommitUser `json:"author"`
+	Committer *CommitUser `json:"committer"`
+	Message   string      `json:"message"`
+	Tree      *CommitMeta `json:"tree"`
+}
+
+// Commit contains information generated from a Git commit.
+type Commit struct {
+	*CommitMeta
+	HTMLURL    string        `json:"html_url"`
+	RepoCommit *RepoCommit   `json:"commit"`
+	Author     *User         `json:"author"`
+	Committer  *User         `json:"committer"`
+	Parents    []*CommitMeta `json:"parents"`
+}
+
+func (c *Client) GetSingleCommit(user, repo, commitID string) (*Commit, error) {
+	commit := new(Commit)
+	return commit, c.getParsedResponse("GET", fmt.Sprintf("/repos/%s/%s/commits/%s", user, repo, commitID), nil, nil, &commit)
+}
+
+func (c *Client) GetReferenceSHA(user, repo, ref string) (string, error) {
+	data, err := c.getResponse("GET", fmt.Sprintf("/repos/%s/%s/commits/%s", user, repo, ref),
+		http.Header{"Accept": []string{MediaApplicationSHA}}, nil)
+	return string(data), err
+}