Add config option to disable "Update branch by rebase" (#18745)

models/repo/repo_unit.go

@@ -115,12 +115,15 @@ type PullRequestsConfig struct {
 	AllowSquash                   bool
 	AllowManualMerge              bool
 	AutodetectManualMerge         bool
+	AllowRebaseUpdate             bool
 	DefaultDeleteBranchAfterMerge bool
 	DefaultMergeStyle             MergeStyle
 }
 
 // FromDB fills up a PullRequestsConfig from serialized format.
 func (cfg *PullRequestsConfig) FromDB(bs []byte) error {
+	// AllowRebaseUpdate = true as default for existing PullRequestConfig in DB
+	cfg.AllowRebaseUpdate = true
 	return json.UnmarshalHandleDoubleEncode(bs, &cfg)
 }