Include assert.h (disabled).

Include assert.h (disabled).
(path_concat): New function.
(do_copy): Use path_concat instead of open-coding part of its
functionality in two places.  This has the additional benefit
of avoiding forming destination names like `D//file' when the
destination directory D includes a trailing slash -- as would
happen when cp is used like this:
mkdir D; touch file D/file; cp -i file D/
Reported by François Pinard.