Find a single object and intermediate reference by a revision string.

See man gitrevisions, or http://git-scm.com/docs/git-rev-parse.html#_specifying_revisions for information on the syntax accepted.

In some cases ( @ { < -n>} or <branchname> @ {upstream}), the expression may point to an intermediate reference. When such expressions are being passed in, reference_out will be valued as well.

The returned object should be released with git_object_free and the returned reference with git_reference_free when no longer needed.