It's implemented in revwalk.c so it has access to the revision
walker's commit cache and related functions. The algorithm is the one
used by git, modified so it fits better with the library's functions.
The code was already there, so factor it out and let users push an OID
by giving it a reference name. Only refs to commits are
supported. Annotated tags will throw an error.
It's not unusual to want the walker to act on HEAD, so add a
convencience function for the case that the user doesn't already have
a resolved HEAD reference.
