lib/process: `PipeOutput` always ensures the output has a newline, even if it didn't

Created by: LawnGnome

Basically, PipeOutput uses fmt.Fprintln, which means that command output that doesn't terminate in a newline gets one anyway at EOF.

At the extreme, such as sourcegraph/customer#1006, this means that you can't use the output of a batch spec script that doesn't include a newline somewhere that can't handle whitespace, like a branch name.