In effect, we switch from `path.is_file()` to `!path.is_dir()`. In cases
where process substitution is used, for example, the path can actually
have type "fifo" instead of "file." Even if it's a fifo, we want to
treat it as-if it were a file. The real key here is that we basically
always want to consider a lone argument as a file so long as we know it
isn't a directory. Because a directory is the only thing that will
causes us to (potentially) search more than one thing.
Fixes#2736
4a30819302