Be more conservative about rehash. No functional change is expected
because we were already checking rehash for blocks with updated
hash. But this way the code is simpler and easy to understand.
If we want to disable the automatic hashing assumption of copy,
we need also to remove any previous hash info already stored
in the content file by previous sync commands run without the
nocopy option.
Now a if the file has a zero nanosecond timestamp, it must
match the full pathname.
Also add the new -N option and an new undocumented test-nano
command to set randomly the nanosecond timestamp of files
that have it at zero.
