Change the default maxPerPage for gitbucket (go-gitea#30392)

This patch improves the migration from gitbucket to gitea.

The gitbucket uses it's own internal perPage value (= 25) for paging and
ignore per_page arguments in the requested URL. This cause gitea to
migrate only 25 issues and 25 PRs from gitbucket repository. This may
not happens on old gitbucket. But recent gitbucket 4.40 or 4.38.4 has
this problem.

This patch change to use this internally hardcoded perPage of gitbucket
as gitea's maxPerPage numer when migrating from gitbucket. There are
several perPage values in gitbucket like 25 for Isseus/PRs and 10 for
Releases. Some of those API doesn't support paging yet. It sounds
difficult to implement, but using the minimum number among them worked
out very well. So, I use 10 in this patch.

Brief descriptions of problems and this patch are also available in
go-gitea#30316.

In addition, I'm not sure what kind of test cases are possible to write
here. It's a test for migration, so it requires testing gitbucket server
and gitea server, I guess. Please let me know if it is possible to write
such test cases here. Thanks!

services/migrations/gitbucket.go

@@ -72,6 +72,11 @@ func (g *GitBucketDownloader) LogString() string {
 // NewGitBucketDownloader creates a GitBucket downloader
 func NewGitBucketDownloader(ctx context.Context, baseURL, userName, password, token, repoOwner, repoName string) *GitBucketDownloader {
 	githubDownloader := NewGithubDownloaderV3(ctx, baseURL, userName, password, token, repoOwner, repoName)
+	// Gitbucket 4.40 uses different internal hard-coded perPage values.
+	// Issues, PRs, and other major parts use 25.  Release page uses 10.
+	// Some API doesn't support paging yet.  Sounds difficult, but using
+	// minimum number among them worked out very well.
+	githubDownloader.maxPerPage = 10
 	githubDownloader.SkipReactions = true
 	githubDownloader.SkipReviews = true
 	return &GitBucketDownloader{