exchangev2: add progress bar around manifest scanning

This can take a long time on large repositories. Let's add a progress
bar so we don't have long periods where it isn't obvious what is
going on.

Differential Revision: https://phab.mercurial-scm.org/D4859

mercurial/exchangev2.py

@@ -320,20 +320,26 b' def _derivefilesfrommanifests(repo, mani'
     ml = repo.manifestlog
     fnodes = collections.defaultdict(dict)
 
+    progress = repo.ui.makeprogress(
+        _('scanning manifests'), total=len(manifestnodes))
+
+    with progress:
     for manifestnode in manifestnodes:
         m = ml.get(b'', manifestnode)
 
         # TODO this will pull in unwanted nodes because it takes the storage
-        # delta into consideration. What we really want is something that takes
-        # the delta between the manifest's parents. And ideally we would
-        # ignore file nodes that are known locally. For now, ignore both
-        # these limitations. This will result in incremental fetches requesting
-        # data we already have. So this is far from ideal.
+            # delta into consideration. What we really want is something that
+            # takes the delta between the manifest's parents. And ideally we
+            # would ignore file nodes that are known locally. For now, ignore
+            # both these limitations. This will result in incremental fetches
+            # requesting data we already have. So this is far from ideal.
         md = m.readfast()
 
         for path, fnode in md.items():
             fnodes[path].setdefault(fnode, manifestnode)
 
+            progress.increment()
+
     return fnodes
 
 def _fetchfiles(repo, tr, remote, fnodes, linkrevs):