--- a/mercurial/copies.py Thu Jun 20 10:58:14 2019 -0700
+++ b/mercurial/copies.py Wed Jun 26 05:20:02 2019 -0700
@@ -268,6 +268,10 @@
# 'work' contains 3-tuples of a (revision number, parent number, copies).
# The parent number is only used for knowing which parent the copies dict
# came from.
+ # NOTE: To reduce costly copying the 'copies' dicts, we reuse the same
+ # instance for *one* of the child nodes (the last one). Once an instance
+ # has been put on the queue, it is thus no longer safe to modify it.
+ # Conversely, it *is* safe to modify an instance popped off the queue.
work = [(r, 1, {}) for r in roots]
heapq.heapify(work)
alwaysmatch = match.always()