tests: prevent enormous output spew in test-lfs-largefiles.t

This test currently fails on Python 3, but in a super-slow way. Adding
this `head` invocation costs us nothing, but will prevent failures in
this area from being super expensive.

Differential Revision: https://phab.mercurial-scm.org/D2600

tests/test-lfs-largefiles.t

@@ -338,7 +338,10 b' locally.'
 No diffs when comparing merge and p1 that kept p1's changes.  Diff of lfs to
 largefiles no longer operates in standin files.
 
-  $ hg diff -r 2:3
+This `head -n 20` looks dumb (since we expect no output), but if something
+breaks you can get 1048576 lines of +y in the output, which takes a looooooong
+time to print.
+  $ hg diff -r 2:3 | head -n 20
   $ hg diff -r 2:6
   diff -r e989d0fa3764 -r 752e3a0d8488 large.bin
   --- a/large.bin	Thu Jan 01 00:00:00 1970 +0000