Write the old object id from the RefUpdate to the
ORIG_HEAD file after the update completes.
Add two new convenience methods to Repository to read
and write the ORIG_HEAD reference similar to the methods
for reading/writing CHERRY_PICK_HEAD and MERGE_HEAD.
Bug: 375525
Change-Id: I120b3b2cd3b1ddae88fce435285bae15cbf96f5e