Avoid double-counting source files in 'tahoe cp --verbose'. fixes #1783

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>
Improve 'tahoe put --help' to clarify behaviour for mutable files, and
make sure the documented behaviour is tested. fixes #1372

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>