lfs: defer registering the pre-push hook until blobs are committed

The hook searches outgoing commits for blobs, and uploads them before letting
the push occur.  No reason to search for that which isn't there.

hgext/lfs/__init__.py

@@ -160,9 +160,6 b' def reposetup(ui, repo):'
     repo.svfs.lfslocalblobstore = blobstore.local(repo)
     repo.svfs.lfsremoteblobstore = blobstore.remote(repo)
 
-    # Push hook
-    repo.prepushoutgoinghooks.add('lfs', wrapper.prepush)
-
     class lfsrepo(repo.__class__):
         @localrepo.unfilteredmethod
         def commitctx(self, ctx, error=False):
@@ -185,10 +182,13 b' def reposetup(ui, repo):'
                 if any(ctx[f].islfs() for f in ctx.files() if f in ctx):
                     repo.requirements.add('lfs')
                     repo._writerequirements()
+                    repo.prepushoutgoinghooks.add('lfs', wrapper.prepush)
                     break
 
         ui.setconfig('hooks', 'commit.lfs', checkrequireslfs, 'lfs')
         ui.setconfig('hooks', 'pretxnchangegroup.lfs', checkrequireslfs, 'lfs')
+    else:
+        repo.prepushoutgoinghooks.add('lfs', wrapper.prepush)
 
 def _trackedmatcher(repo, ctx):
     """Return a function (path, size) -> bool indicating whether or not to