phabricator: teach `getoldnodedrevmap()` to handle folded reviews The tricky part here is reasoning through all of the possible predecessor scenarios. In the typical case of submitting a folded range and then resubmitting it (also folded), filtering the list of commits for the diff stored on Phabricator through the local predecessor list for each single node will result in the typical 1:1 mapping to the old node. There are edge cases like using `hg fold` within the range prior to resubmitting, that will result in mapping to multiple old nodes. In that case, the first direct predecessor is needed for the base of the diff, and the last direct predecessor is needed for the head of the diff in order to make sure that the entire range is included in the diff content. And none of this matters for commits in the middle of the range, as they are never used. Fortunately the only crucial thing here is the `drev` number for each node. For these complicated cases where there are multiple old nodes, simply ignore them all. This will cause `createdifferentialrevision()` to generate a new diff (within the same Differential), and avoids complicating the code. Differential Revision: https://phab.mercurial-scm.org/D8311