Prevent the first change in a continuous _changes feed from being stripped by open_pipe

I noticed the current open_pipe command will not emit the first change from a changes feed if it's continuous.

This happens because the continuous feed has a 'one revision per line' layout, rather than the usual, so assuming the first line of the response is a header leads to losing a line of your changes.

Not quite sure how to write a test for this at the moment - sorry!