feat: add commit limit for webhook payload (#6797)

- Adds a new option `[webhook].PAYLOAD_COMMIT_LIMIT` that limits the amount of commits is sent for each webhook payload, this was previously done via `[ui].FEED_MAX_COMMIT_NUM` which feels incorrect.
- The default is 15 for this new option, purely arbitary.
- Resolves forgejo/forgejo#6780
- Added unit testing, it's quite a lot because this the notification
area is not really easy to test and rather should've been a integration test
but that ends up having more complicated than trying doing an unit test.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6797
Reviewed-by: Otto <otto@codeberg.org>
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

services/repository/push.go

@@ -252,10 +252,6 @@ func pushUpdates(optsList []*repo_module.PushUpdateOptions) error {
 					commits.CompareURL = ""
 				}
 
-				if len(commits.Commits) > setting.UI.FeedMaxCommitNum {
-					commits.Commits = commits.Commits[:setting.UI.FeedMaxCommitNum]
-				}
-
 				notify_service.PushCommits(ctx, pusher, repo, opts, commits)
 
 				// Cache for big repository