diff -r 3e7387337a3c -r 7e66e7999bdd mercurial/exchange.py
--- a/mercurial/exchange.py	Mon Jul 02 18:24:26 2018 -0700
+++ b/mercurial/exchange.py	Mon Jul 02 18:32:20 2018 -0700
@@ -15,6 +15,7 @@
     bin,
     hex,
     nullid,
+    nullrev,
 )
 from .thirdparty import (
     attr,
@@ -23,6 +24,7 @@
     bookmarks as bookmod,
     bundle2,
     changegroup,
+    dagutil,
     discovery,
     error,
     lock as lockmod,
@@ -1875,6 +1877,131 @@
         new_args['excludepats'] = req_excludes
     return new_args
 
+def _computeellipsis(repo, common, heads, known, match, depth=None):
+    """Compute the shape of a narrowed DAG.
+
+    Args:
+      repo: The repository we're transferring.
+      common: The roots of the DAG range we're transferring.
+              May be just [nullid], which means all ancestors of heads.
+      heads: The heads of the DAG range we're transferring.
+      match: The narrowmatcher that allows us to identify relevant changes.
+      depth: If not None, only consider nodes to be full nodes if they are at
+             most depth changesets away from one of heads.
+
+    Returns:
+      A tuple of (visitnodes, relevant_nodes, ellipsisroots) where:
+
+        visitnodes: The list of nodes (either full or ellipsis) which
+                    need to be sent to the client.
+        relevant_nodes: The set of changelog nodes which change a file inside
+                 the narrowspec. The client needs these as non-ellipsis nodes.
+        ellipsisroots: A dict of {rev: parents} that is used in
+                       narrowchangegroup to produce ellipsis nodes with the
+                       correct parents.
+    """
+    cl = repo.changelog
+    mfl = repo.manifestlog
+
+    cldag = dagutil.revlogdag(cl)
+    # dagutil does not like nullid/nullrev
+    commonrevs = cldag.internalizeall(common - set([nullid])) | set([nullrev])
+    headsrevs = cldag.internalizeall(heads)
+    if depth:
+        revdepth = {h: 0 for h in headsrevs}
+
+    ellipsisheads = collections.defaultdict(set)
+    ellipsisroots = collections.defaultdict(set)
+
+    def addroot(head, curchange):
+        """Add a root to an ellipsis head, splitting heads with 3 roots."""
+        ellipsisroots[head].add(curchange)
+        # Recursively split ellipsis heads with 3 roots by finding the
+        # roots' youngest common descendant which is an elided merge commit.
+        # That descendant takes 2 of the 3 roots as its own, and becomes a
+        # root of the head.
+        while len(ellipsisroots[head]) > 2:
+            child, roots = splithead(head)
+            splitroots(head, child, roots)
+            head = child  # Recurse in case we just added a 3rd root
+
+    def splitroots(head, child, roots):
+        ellipsisroots[head].difference_update(roots)
+        ellipsisroots[head].add(child)
+        ellipsisroots[child].update(roots)
+        ellipsisroots[child].discard(child)
+
+    def splithead(head):
+        r1, r2, r3 = sorted(ellipsisroots[head])
+        for nr1, nr2 in ((r2, r3), (r1, r3), (r1, r2)):
+            mid = repo.revs('sort(merge() & %d::%d & %d::%d, -rev)',
+                            nr1, head, nr2, head)
+            for j in mid:
+                if j == nr2:
+                    return nr2, (nr1, nr2)
+                if j not in ellipsisroots or len(ellipsisroots[j]) < 2:
+                    return j, (nr1, nr2)
+        raise error.Abort(_('Failed to split up ellipsis node! head: %d, '
+                            'roots: %d %d %d') % (head, r1, r2, r3))
+
+    missing = list(cl.findmissingrevs(common=commonrevs, heads=headsrevs))
+    visit = reversed(missing)
+    relevant_nodes = set()
+    visitnodes = [cl.node(m) for m in missing]
+    required = set(headsrevs) | known
+    for rev in visit:
+        clrev = cl.changelogrevision(rev)
+        ps = cldag.parents(rev)
+        if depth is not None:
+            curdepth = revdepth[rev]
+            for p in ps:
+                revdepth[p] = min(curdepth + 1, revdepth.get(p, depth + 1))
+        needed = False
+        shallow_enough = depth is None or revdepth[rev] <= depth
+        if shallow_enough:
+            curmf = mfl[clrev.manifest].read()
+            if ps:
+                # We choose to not trust the changed files list in
+                # changesets because it's not always correct. TODO: could
+                # we trust it for the non-merge case?
+                p1mf = mfl[cl.changelogrevision(ps[0]).manifest].read()
+                needed = bool(curmf.diff(p1mf, match))
+                if not needed and len(ps) > 1:
+                    # For merge changes, the list of changed files is not
+                    # helpful, since we need to emit the merge if a file
+                    # in the narrow spec has changed on either side of the
+                    # merge. As a result, we do a manifest diff to check.
+                    p2mf = mfl[cl.changelogrevision(ps[1]).manifest].read()
+                    needed = bool(curmf.diff(p2mf, match))
+            else:
+                # For a root node, we need to include the node if any
+                # files in the node match the narrowspec.
+                needed = any(curmf.walk(match))
+
+        if needed:
+            for head in ellipsisheads[rev]:
+                addroot(head, rev)
+            for p in ps:
+                required.add(p)
+            relevant_nodes.add(cl.node(rev))
+        else:
+            if not ps:
+                ps = [nullrev]
+            if rev in required:
+                for head in ellipsisheads[rev]:
+                    addroot(head, rev)
+                for p in ps:
+                    ellipsisheads[p].add(rev)
+            else:
+                for p in ps:
+                    ellipsisheads[p] |= ellipsisheads[rev]
+
+    # add common changesets as roots of their reachable ellipsis heads
+    for c in commonrevs:
+        for head in ellipsisheads[c]:
+            addroot(head, c)
+    return visitnodes, relevant_nodes, ellipsisroots
+
 def caps20to10(repo, role):
     """return a set with appropriate options to use bundle20 during getbundle"""
     caps = {'HG20'}