Sorry for the late reply.

To work around this situation, you can remove the pending patch
(_darcs/patches/pending).

The hanging is because the repository is still using the old darcs 1 format (you
would need to convert to the darcs 2 format to avoid the exponential merge problem).

The reason there was an exponential merge was because of the mv A -> B; rm A;
add B local changes.  I believe these were caused by a bug in handling the
pending patch -- one that has been fixed.

I'm marking this as resolved.  If anybody can show me an instance of this bug
being creating in darcs 2, please open a new issue.