Api.RefA Ref as an indirect way of referring to a commit.
t is a regular git ref or a merge request record of mr_info.
Id as a regular git ref or a merge request.
This is intended to be used by a user when a t type is manipulated by the API.
val pp : t Fmt.tPretty print t.
val to_git : t -> stringto_git t is the Git-format string of the ref, e.g."refs/pull/%d/head"