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

phabricator: teach createdifferentialrevision() to allow a folded commit range No visible changes here, until an option to enable it is added to `phabsend`. Differential Revision: https://phab.mercurial-scm.org/D8310

phabricator: combine commit messages into the review when folding commits No visible changes here, until an option to enable it is added to `phabsend`. This combines the Differential fields like Arcanist does, rather than simply concatenating the text blocks. Aside from populating everything properly in the web interface, Phabricator fails the review create/update if repeated fields are seen as would happen with simple concatenation. On the flip side, now that the Summary and Test Plan fields can contain data from multiple commits, we can't just join these fields together to determine if an amend is needed. If that were to happen, every single commit in the folded range would get amended with the combined commit message, which seems clearly wrong. Aside from making a minor assumption about the content of the Differential Revision field (it seems they allow some minor variances with spacing), this means that for folded reviews, you can't post it, go to the web page add a missing Test Plan, and then get it added to the commit message by re-posting it. I don't think that's a big deal. Differential Revision: https://phab.mercurial-scm.org/D8309