ch-grow: COPY fails if directories already exist

If you try to COPY a directory tree that already exists in part at the destination, then ch-grow crashes. E.g., consider this instruction:

COPY --from=buildstage /hello/usr/local/share /usr/local/share

This is attempting to merge two directory hierarchies. The destination contains subdirectories applications, info, and man, while the source contains info, locale, and man.

Docker is happy to execute this, and I haven't analyzed precisely what happens.

ch-grow crashes with:

FileExistsError: [Errno 17] File exists: '/var/tmp/reidpr/ch-grow/img/multistage//usr/local/share/man'