tests/test-gendoc-sv.t

unbundle: add a small comment to clarify the 'check_heads' call

Bundle2 has its own mechanisms to check for heads (and other) changes, so push
using bundle2 is relying on the "check:heads" bundle part of unbundle and the
'check_heads' call is not checking anything. We add a small comment to make
this clearer.

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors