parser: Add support for rename-only patches

Yann E. MORIN reported a problem where patchwork drops patches with only
renames:

 http://lists.busybox.net/pipermail/buildroot/2013-October/079999.html

This change fixes the issue by adding a new state for patch 'meta'
headers: header test which is not parsed as a hunk, but has the same
behaviour as a hunk in that it will cause the patchbuf to be populated.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>