changegroup: add first logic to send file header

This will allow us to later change how we filter needed file nodes

mercurial/localrepo.py

@@ -1554,14 +1554,8 b' class localrepository(repo.repository):'
                 # missing.
                 missingfnodes = fnodes.pop(fname, {})
                 prune(filerevlog, missingfnodes)
-                # If any filenodes are left, generate the group for them,
-                # otherwise don't bother.
-                if missingfnodes:
-                    yield changegroup.chunkheader(len(fname))
-                    yield fname
-                    # Create a group generator and only pass in a changenode
-                    # lookup function as we need to collect no information
-                    # from filenodes.
+                first = True
+
                     def flookup(revlog, x):
                         # even though we print the same progress on
                         # most loop iterations, put the progress call
@@ -1572,6 +1566,12 b' class localrepository(repo.repository):'
                         return missingfnodes[x]
 
                     for chunk in filerevlog.group(missingfnodes, flookup):
+                    if first:
+                        if chunk == changegroup.closechunk():
+                            break
+                        yield changegroup.chunkheader(len(fname))
+                        yield fname
+                        first = False
                         yield chunk
             # Signal that no more groups are left.
             yield changegroup.closechunk()
@@ -1645,11 +1645,8 b' class localrepository(repo.repository):'
                 filerevlog = self.file(fname)
                 if not len(filerevlog):
                     raise util.Abort(_("empty or missing revlog for %s") % fname)
+                first = True
                 nodeiter = gennodelst(filerevlog)
-                nodeiter = list(nodeiter)
-                if nodeiter:
-                    yield changegroup.chunkheader(len(fname))
-                    yield fname
                     def flookup(revlog, x):
                         self.ui.progress(
                             _('bundling'), idx, item=fname,
@@ -1657,6 +1654,12 b' class localrepository(repo.repository):'
                         return cl.node(revlog.linkrev(revlog.rev(x)))
 
                     for chunk in filerevlog.group(nodeiter, flookup):
+                    if first:
+                        if chunk == changegroup.closechunk():
+                            break
+                        yield changegroup.chunkheader(len(fname))
+                        yield fname
+                        first = False
                         yield chunk
             self.ui.progress(_('bundling'), None)