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

hgext/phabricator.py

@@ -483,18 +483,23 b' def getoldnodedrevmap(repo, nodelist):'
         alldiffs = callconduit(
             unfi.ui, b'differential.querydiffs', {b'revisionIDs': drevs}
         )
-        getnode = lambda d: bin(getdiffmeta(d).get(b'node', b'')) or None
+
+        def getnodes(d, precset):
+            # Ignore other nodes that were combined into the Differential
+            # that aren't predecessors of the current local node.
+            return [n for n in getlocalcommits(d) if n in precset]
+
         for newnode, (force, precset, drev) in toconfirm.items():
             diffs = [
                 d for d in alldiffs.values() if int(d[b'revisionID']) == drev
             ]
 
-            # "precursors" as known by Phabricator
-            phprecset = {getnode(d) for d in diffs}
+            # local predecessors known by Phabricator
+            phprecset = {n for d in diffs for n in getnodes(d, precset)}
 
             # Ignore if precursors (Phabricator and local repo) do not overlap,
             # and force is not set (when commit message says nothing)
-            if not force and not bool(phprecset & precset):
+            if not force and not phprecset:
                 tagname = b'D%d' % drev
                 tags.tag(
                     repo,
@@ -519,7 +524,20 b' def getoldnodedrevmap(repo, nodelist):'
             oldnode = lastdiff = None
             if diffs:
                 lastdiff = max(diffs, key=lambda d: int(d[b'id']))
-                oldnode = getnode(lastdiff)
+                oldnodes = getnodes(lastdiff, precset)
+
+                # If this commit was the result of `hg fold` after submission,
+                # and now resubmitted with --fold, the easiest thing to do is
+                # to leave the node clear.  This only results in creating a new
+                # diff for the _same_ Differential Revision if this commit is
+                # the first or last in the selected range.
+                # If this commit is the result of `hg split` in the same
+                # scenario, there is a single oldnode here (and multiple
+                # newnodes mapped to it).  That makes it the same as the normal
+                # case, as the edges of the newnode range cleanly maps to one
+                # oldnode each.
+                if len(oldnodes) == 1:
+                    oldnode = oldnodes[0]
                 if oldnode and not has_node(oldnode):
                     oldnode = None
 
@@ -1667,6 +1685,21 b' def get_amended_desc(drev, ctx, folded):'
     return _differentialrevisiondescre.sub(uri, ctx.description())
 
 
+def getlocalcommits(diff):
+    """get the set of local commits from a diff object
+
+    See ``getdiffmeta()`` for an example diff object.
+    """
+    props = diff.get(b'properties') or {}
+    commits = props.get(b'local:commits') or {}
+    if len(commits) > 1:
+        return {bin(c) for c in commits.keys()}
+
+    # Storing the diff metadata predates storing `local:commits`, so continue
+    # to use that in the --no-fold case.
+    return {bin(getdiffmeta(diff).get(b'node', b'')) or None}
+
+
 def getdiffmeta(diff):
     """get commit metadata (date, node, user, p1) from a diff object