Force-push to base repo's ref/pull/#/head (#3393)

* Force-push to base repo's ref/pull/#/head

Fixes force-pushing to pull request branches, otherwise failing with:

[...gitea/models/pull.go:1022 AddTestPullRequestTask()] [E] PushToBaseRepo: Push: exit status 1 - To /path/to/gitea-repositories/org/repo.git
 ! [rejected]        issue-fix -> refs/pull/20/head (non-fast-forward)

* format

models/pull.go

@@ -1010,6 +1010,7 @@ func (pr *PullRequest) PushToBaseRepo() (err error) {
 	if err = git.Push(headRepoPath, git.PushOptions{
 		Remote: tmpRemoteName,
 		Branch: fmt.Sprintf("%s:%s", pr.HeadBranch, headFile),
+		Force:  true,
 	}); err != nil {
 		return fmt.Errorf("Push: %v", err)
 	}