Vcs.RevSourceA revision uniquely identifies a node in the dag formed by the commits of a repository. The name was inherited from Mercurial. For git, this correspond to the commit-hash. In both systems, these are 40-chars hashes.
Given that t = string in the implementation, this function is just the identity.
of_string str returns Ok str if X.invariant str = true, and an error otherwise. This is meant to be used to validate untrusted entries.