verify: extract "manifest" constant into variable
The "manifest" label that's used in error messages will instead be the
directory path for subdirectory manifests (not the root manifest), so
let's extract the constant to a variable already to make future
patches simpler.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html