Fix migration from gitbucket (repost) (#22477)

Reposting pull request for #22465 

> Migration from GitBucket does not work due to a access for "Reviews"
API on GitBucket that makes 404 response. This PR has following changes.
> 1. Made to stop access for Reviews API while migrating from GitBucket.
> 2. Added support for custom URL (e.g.
`http://example.com/gitbucket/owner/repository`)
> 3. Made to accept for git checkout URL
(`http://example.com/git/owner/repository.git`)

Co-authored-by: zeripath <art27@cantab.net>

services/migrations/gitbucket.go

@@ -33,10 +33,14 @@ func (f *GitBucketDownloaderFactory) New(ctx context.Context, opts base.MigrateO
 		return nil, err
 	}
 
-	baseURL := u.Scheme + "://" + u.Host
 	fields := strings.Split(u.Path, "/")
-	oldOwner := fields[1]
-	oldName := strings.TrimSuffix(fields[2], ".git")
+	if len(fields) < 2 {
+		return nil, fmt.Errorf("invalid path: %s", u.Path)
+	}
+	baseURL := u.Scheme + "://" + u.Host + strings.TrimSuffix(strings.Join(fields[:len(fields)-2], "/"), "/git")
+
+	oldOwner := fields[len(fields)-2]
+	oldName := strings.TrimSuffix(fields[len(fields)-1], ".git")
 
 	log.Trace("Create GitBucket downloader. BaseURL: %s RepoOwner: %s RepoName: %s", baseURL, oldOwner, oldName)
 	return NewGitBucketDownloader(ctx, baseURL, opts.AuthUsername, opts.AuthPassword, opts.AuthToken, oldOwner, oldName), nil
@@ -71,6 +75,7 @@ func (g *GitBucketDownloader) ColorFormat(s fmt.State) {
 func NewGitBucketDownloader(ctx context.Context, baseURL, userName, password, token, repoOwner, repoName string) *GitBucketDownloader {
 	githubDownloader := NewGithubDownloaderV3(ctx, baseURL, userName, password, token, repoOwner, repoName)
 	githubDownloader.SkipReactions = true
+	githubDownloader.SkipReviews = true
 	return &GitBucketDownloader{
 		githubDownloader,
 	}

services/migrations/github.go

@@ -76,6 +76,7 @@ type GithubDownloaderV3 struct {
 	curClientIdx  int
 	maxPerPage    int
 	SkipReactions bool
+	SkipReviews   bool
 }
 
 // NewGithubDownloaderV3 creates a github Downloader via github v3 API
@@ -809,6 +810,9 @@ func (g *GithubDownloaderV3) convertGithubReviewComments(cs []*github.PullReques
 // GetReviews returns pull requests review
 func (g *GithubDownloaderV3) GetReviews(reviewable base.Reviewable) ([]*base.Review, error) {
 	allReviews := make([]*base.Review, 0, g.maxPerPage)
+	if g.SkipReviews {
+		return allReviews, nil
+	}
 	opt := &github.ListOptions{
 		PerPage: g.maxPerPage,
 	}