fastmanifest: fallback to old manifest in readshallowfast

Summary:
fastmanifest makes bundling slower because fastmanifest `readshallowfast` always returns full manifest.
That's a big problem for infinitepush.
Let's copy-paste readshallowfast implementation from upstream. It uses readshallowdelta() if possible.

Test Plan:
1) Run all the tests for fb-hgext
2) Run infinitepush with this extension enabled. Make it is fast

Reviewers: durham, simonfar, rmcelroy, quark

Reviewed By: quark

Subscribers: mjpieters, #sourcecontrol

Differential Revision: https://phabricator.intern.facebook.com/D4088360

Tasks: 13907166

Signature: t1:4088360:1477580931:746e4054380403abbc52d1922583021b81f31bb6

fastmanifest/__init__.py

@@ -221,7 +221,7 @@ class FastManifestExtension(object):
         extensions.wrapfunction(manifest.manifest, 'read', factory.read)
         try:
             extensions.wrapfunction(manifest.manifest, 'readshallowfast',
-                                    factory.read)
+                                    factory.readshallowfast)
         except AttributeError:
             # The function didn't use to be defined in previous versions
             # of hg

fastmanifest/implementation.py

@@ -779,6 +779,23 @@ class manifestfactory(object):
                               loadflat=lambda: orig(*args, **kwargs),
                               node=args[1])
 
+    def readshallowfast(self, orig, *args, **kwargs):
+        # copy-paste from manifest.readshallowfast
+        manifest = args[0]
+        if len(args) == 2:
+            node = args[1]
+        else:
+            node = kwargs['node']
+        r = manifest.rev(node)
+        deltaparent = manifest.deltaparent(r)
+        if (deltaparent != revlog.nullrev and
+                deltaparent in manifest.parentrevs(r)):
+            return manifest.readshallowdelta(node)
+        return hybridmanifest(self.ui,
+                              args[0].opener,
+                              loadflat=lambda: orig(*args, **kwargs),
+                              node=args[1])
+
     def newgetitem(self, orig, *args, **kwargs):
         # args[0] == instance of manifestlog
         # args[1] = node