Add `retries` option to clone manifest

Will be used to support https://github.com/drone/drone-git/pull/43

manifest/clone.go

@@ -8,6 +8,7 @@ package manifest
 type Clone struct {
 	Disable    bool `json:"disable,omitempty"`
 	Depth      int  `json:"depth,omitempty"`
+	Retries    int  `json:"retries,omitempty"`
 	SkipVerify bool `json:"skip_verify,omitempty" yaml:"skip_verify"`
 	Trace      bool `json:"trace,omitempty"`
 }