R1:7c502e3e4eda

Fix performance issue with large changesets in Differential

Summary:
Fixes T3151. Javelin treats a behavior without parameters as a global behavior and invokes it only once no matter how many times it is initialized (this is necessarily correct for any reasonable behavior, as the inputs do not vary). A recent patch changed `differential-dropdown-menus` from a zero-argument global behavior to an implicitly nonzero-argument behavior by adding `pht`.

Currently, we initialize the behavior next to dropdown…