dgit: Include quilt mode in archive/ tag in split view

dgit: Include quilt mode in archive/ tag in split view

After this, a program examining this tag can see one of the following
possibilities:

"split --quilt=<splitting>"
   splitting quilt view, `3.0 (quilt)'

"split --quilt=<nonsplitting>"
   `3.0 (quilt)', mostly-dgit-compatible branch, but split view
   (bureaucracy commits not on maintainer branch)

"no-split --quilt=<nonsplitting>"
   `3.0 (quilt)', maintainer uses dgit branch

"split" / "no-split"
   non-`3.0 (quilt)'

No "split" or "no-split"
   generated by old versions of dgit, quilt transformation
   information not captured in machine-readable form

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>