Patch differential-dataflow to frankmcsherry/vec_merger_patch

Apply upstream fix from
TimelyDataflow/differential-dataflow#701
to address performance regressions in the merge batcher.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antiguru

Cargo.lock

@@ -343,6 +343,10 @@ rdkafka-sys = { git = "https://github.com/MaterializeInc/rust-rdkafka.git" }
 # Note: All changes in our fork of tiberius should be pushed to the `mz_changes` branch.
 tiberius = { git = "https://github.com/MaterializeInc/tiberius", rev="64ca594cc22ed67d072c2d0110455da50539e1cd" }
 
+# Waiting on https://github.com/TimelyDataflow/differential-dataflow/pull/701.
+differential-dataflow = { git = "https://github.com/frankmcsherry/differential-dataflow.git", branch = "vec_merger_patch" }
+differential-dogs3 = { git = "https://github.com/frankmcsherry/differential-dataflow.git", branch = "vec_merger_patch" }
+
 # Allows us to use bzip2-sys rather than the rust reimpl.
 # All changes should go to the `mz_changes` branch.
 # Once https://github.com/Nullus157/async-compression/pull/364 lands we can remove this.