Use fmt.Sprintf correctly (#17886)

contrib/pr/checkout.go

 		//Use git cli command for windows
 		runCmd("git", "fetch", remoteUpstream, fmt.Sprintf("pull/%s/head:%s", pr, branch))
 	} else {
-		ref := fmt.Sprintf(gitea_git.PullPrefix+"%s/head:%s", pr, branchRef)
+		ref := fmt.Sprintf("%s%s/head:%s", gitea_git.PullPrefix, pr, branchRef)
 		err = repo.Fetch(&git.FetchOptions{
 			RemoteName: remoteUpstream,
 			RefSpecs: []config.RefSpec{

models/pull.go

 
 // GetGitRefName returns git ref for hidden pull request branch
 func (pr *PullRequest) GetGitRefName() string {
-	return fmt.Sprintf(git.PullPrefix+"%d/head", pr.Index)
+	return fmt.Sprintf("%s%d/head", git.PullPrefix, pr.Index)
 }
 
 // IsChecking returns true if this pull request is still checking conflict.

modules/convert/pull.go

 		},
 		Head: &api.PRBranchInfo{
 			Name:   pr.HeadBranch,
-			Ref:    fmt.Sprintf(git.PullPrefix+"%d/head", pr.Index),
+			Ref:    fmt.Sprintf("%s%d/head", git.PullPrefix, pr.Index),
 			RepoID: -1,
 		},
 	}

modules/migration/pullrequest.go

 
 // GetGitRefName returns pull request relative path to head
 func (p PullRequest) GetGitRefName() string {
-	return fmt.Sprintf(git.PullPrefix+"%d/head", p.Number)
+	return fmt.Sprintf("%s%d/head", git.PullPrefix, p.Number)
 }
 
 // PullRequestBranch represents a pull request branch