diff on non existent channel:

$ pijul init p1
$ cd p1
$ touch a
$ pijul add a
$ pijul rec -am"."
Hash: 2KHVZEYI5B3WGVNH733AJNGOEZ42R7XPZXAJANRFSBS2ZLEWBZEAC
$ pijul diff --channel notfound
message = ''
timestamp = '2023-03-28T05:13:52.830341035Z'
authors = []

# Hunks

1. Root add
  up 1.0, new 1:1

2. File addition: "a" in "" "UTF-8"
  up 0.2, new 6:31
$ pijul channel
* main
$