tests/test-gendoc-sv.t

repository: introduce constant for sparse repo requirement and use it

In future we will like to much cleaner logic around which requirement is for
working copy and which can go in store. To start with that, we first need to
de-clutter the requirement values spread around and replace them with constants.

Differential Revision: https://phab.mercurial-scm.org/D8911

#require docutils gettext

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