Updates for a faster link command.

 - Don't store intermediate `paths` variable; use pipes.
 - Use process substitution to speed up loops.
 - Minor clean-up & fix a few shellcheck warnings. 
 - Linking files with a newline is not actually fixed, but the
   behavior did change slightly, so I updated the test.
It may be good to consider logging everything in homeshick to `stderr` instead of most things to `stdout`, and only errors to `stderr` (from what I remember of my brief look at `log.sh`, I think that is the current behavior). Logging to stderr in bash scripts enables you to log things without polluting `stdout`, which often gets piped around or evaluated by other functions. I would be happy to do the work if you feel like it is a good idea as well.