Fix has_changed with large diff output

[lstoll]

has_changed uses grep with the quiet flag, which returns after the first match
is found. This causes issues with input that has a large number of lines and a
match that is triggered early, as grep returns and causes the echo to fail
writing to the pipe. Echo's exit status is used for the test because the
pipefail option is set, ultimately causing the change to be skipped.

Fix this by removing the echo, and replacing it with a herestring.

[lstoll]

@chronotc any thoughts on landing this fix?

[chronotc]

Thanks for the enhancement @lstoll. Sorry for taking so long