Detect changes after `fclones group` to avoid copying the wrong data

[th1000s]

If files change after an fclones group run without updating the timestamps and remain the same size, then the fclones link command (and others) can lead to data loss:

$ mkdir z; cd z; echo same > 1; echo same > 2; echo abcd > Z
$ cat ?
same
same
abcd
$ fclones group . -o log 
$ cp -a Z 1   # timestamp is kept
$ fclones link < log
$ cat ?
abcd
abcd
abcd

This could be avoided by also checking that the ctime of a file is older than the start of the group run, and if not re-checking or aborting.

Maybe even add a --paranoid option to check the content bite-by-byte before acting on it. But even in this case I am not aware of any (Unix) way to guarantee exclusive write access to a file, so maybe mention that the checked data is expected to not change.