Add force_merge to merge request and fix checking mergable (#23010)

Fix #23000.

services/forms/repo_form.go

@@ -604,7 +604,7 @@ type MergePullRequestForm struct {
 	MergeMessageField      string
 	MergeCommitID          string // only used for manually-merged
 	HeadCommitID           string `json:"head_commit_id,omitempty"`
-	ForceMerge             *bool  `json:"force_merge,omitempty"`
+	ForceMerge             bool   `json:"force_merge,omitempty"`
 	MergeWhenChecksSucceed bool   `json:"merge_when_checks_succeed,omitempty"`
 	DeleteBranchAfterMerge bool   `json:"delete_branch_after_merge,omitempty"`
 }