tests: remove a"| head -n 2" that just hides an error code
The output from core used to be longer and it made sense, but no
longer does.
CORE-TEST-OUTPUT-UPDATE: 5199c5b6fd29
--- a/tests/test-options.t Thu Oct 18 22:11:43 2018 -0700
+++ b/tests/test-options.t Thu Oct 18 21:59:24 2018 -0700
@@ -24,6 +24,7 @@
> allowunstable
> exchange
> EOF
- $ hg prune | head -n 2
+ $ hg prune
hg: unknown command 'prune'
(use 'hg help' for a list of commands)
+ [255]