Cloning with jj on Case-Insensitive Filesystems

When this applies

jj git clone (or the bundled jj-clone wrapper) fails with output like:

Fetching into new repo in "..."
git: warning: no common commits
remote: Enumerating objects: ...
Error: Failed to update refs: refs/remotes/origin/<some-ref-name>

The named ref is one half of a case-collision pair on the remote (e.g. MergeMasterToDevelop vs MergeMastertoDevelop).

Root cause