Introduce HG_PREPEND to solve pretxn races - add writepending to flush delayed writes to separate file - add support in hooks for lazy evaluation of callable parameters - add HG_PENDING to pretxn hooks - call writepending if hook is used - pass repo root to hook environment - if HG_PENDING = repo root, we're in pretxn hook - read pending data to make pending changesets visible - filter HG_PENDING in tests/printenv.py