use rvalue-qual Future::unit(): pass 2

Summary:
This is part of "the great r-valuification of folly::Future":

* This is something we should do for safety in general.
* Several of folly::Future's methods are lvalue-qualified even though they act as though they are rvalue-qualified, that is, they provide a postcondition that says, in effect, callers should act as though the method invalidated its `this` object (regardless of whether that invalidation was actual or logical).
* This violates the C++ principle to "Express ideas directly in code" (see Core Guidelines), and generally makes it more confusing for callers as well as hiding the actual semantics from tools (linters, compilers, etc.).
* This dichotomy and confusion has manifested itself by some failures around D7840699 since lvalue-qualification hides that operation's move-out semantics - leads to some use of future operations that are really not correct, but are not obviously incorrect.
* The goal of rvalueification is to make sure methods that are logically rvalue-qualified are actually rvalue-qualified, which forces callsites to acknowledge that rvalueification, e.g., `std::move(f).unit(...)` instead of `f.unit(...)`. This syntactic change in the callsites forces callers to acknowledge the method's rvalue semantics.

Codemod changes:

* expr.unit(...) ==> std::move(expr).unit(...)  // if expr is not already an xvalue
* expr->unit(...) ==> std::move(*expr).unit(...)

Note: operator precedence of that last step is safe - no need to parenthesize `expr`. Reason: `->` binds more tightly than unary `*`.

Reviewed By: LeeHowes

Differential Revision: D9347419

fbshipit-source-id: 2773365f3793d977f2bad1c0a85ef394633e7d2c

eden/fs/inodes/TreeInode.cpp

@@ -2960,7 +2960,7 @@ folly::Future<folly::Unit> TreeInode::loadMaterializedChildren(
     results.emplace_back(
         recurse == Recurse::DEEP
             ? std::move(future).then(recursivelyLoadMaterializedChildren)
-            : future.unit());
+            : std::move(future).unit());
   }
 
   return folly::collectAll(results).unit();