Protocol change: Add distro info to Dgit field

Downstream clients may need this to get the relevant git commits.
Worse, with the new rewrite functionality, they may get the _wrong_
commits because they may not be rewritten.

For now we implement the publication side.  The reader side is
theoretically straightforward, but probably not so simple in reality,
and certainly fiddly to test.

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