Thanks for the patch, Matthew.

I just noticed Lars installed 6f94c2405f4c82b63da19de89549aff1fad7e594
on master, changing erc-generate-new-buffer-name.  Will your patch
require adjusting to accommodate Lars's changes?

Minor nits:

- Please omit the "Signed-off-by:" from your commit messages to Emacs.
- Consider dropping the second "* lisp/erc/erc.el" for (erc-cmd-JOIN),
  since the previous entry (erc-generate-new-buffer-name) was also a
  change to lisp/erc/erc.el.

Thanks.