This is from changeset 249a47720ddcf896a9f07600c429a1b4492b805e from the version-control-tools repo. It contains a fix to restore compatibility with Mercurial 3.7, which caused mozharness tests to fail because that test pins Mercurial 3.7.3 in a requirements file. This is a bug. But getting a modern robustcheckout deployed is more important than upgrading that test.