Previous test did not notice if there is a dangling symlink.
Also, if a directory exists with the same name as the imported file, that
cannot work, so don't let --force have an effect.
Note that the hash backends were made to stop printing a (checksum..)
message as part of this, since it showed up without a file when deciding
whether to act on a file. Should have probably removed that message a while
ago anyway, I suppose.
Note that --deduplicate currently checksums each file twice,
once to see if it's a known key, and once when importing it.
Perhaps this could be revisited and the extra checksum gotten rid of,
at the cost of not locking down the file when adding it.
This avoids some small overhead by only running the check once per command;
it also ensures that, even if the command doesn't find anything to run on,
it still fails to run when in a bare repo.
