merge: add file ancestor linknode to mergestate During a merge, each file has a current commitnode+filenode, an other commitnode+filenode, and an ancestor commitnode+filenode. The ancestor commitnode is not stored though, and we rely on the ability for the filectx() to look up the commitnode by using the filenode's linkrev. In alternative backends (like remotefilelog), linkrevs may have restriction that prevent arbitrary linkrev look up given a filenode. This patch accounts for that by storing the ancestor commitnode in the merge state so that it is available later at resolve time. This results in some test changes because the ancestor commitnode we're using at resolve time changes slightly. Before, we used the linkrev commit, which is the earliest commit that introduced that particular filenode (which may not be the latest common ancestor of the commits being merged). Now we use the latest common ancestor of the merged commits as the commitnode. This is fine though, because that commit contains the same filenode as the linkrev'd commit.