Match api migration behavior to web behavior (#23552) (#23573)

Backport #23552 by @atomaka

When attempting to migrate a repository via the API endpoint comments
are always included. This can create a problem if your source repository
has issues or pull requests but you do not want to import them into
Gitea that displays as something like:

> Error 500: We were unable to perform the request due to server-side
problems. 'comment references non existent IssueIndex 4

There are only two ways to resolve this:
1. Migrate using the web interface
2. Migrate using the API including at issues or pull requests.

This PR matches the behavior of the API migration router to the web
migration router.

Co-authored-by: Andrew Tomaka <atomaka@atomaka.com>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/api/v1/repo/migrate.go

@@ -154,7 +154,7 @@ func Migrate(ctx *context.APIContext) {
 		Issues:         form.Issues,
 		Milestones:     form.Milestones,
 		Labels:         form.Labels,
-		Comments:       true,
+		Comments:       form.Issues || form.PullRequests,
 		PullRequests:   form.PullRequests,
 		Releases:       form.Releases,
 		GitServiceType: gitServiceType,