Tag change: Rename debiantag to debiantag_old

Tag change: Rename debiantag to debiantag_old

We are going to change the tag name of the main signed tag used by
dgit to signal the user's intent to the archive server.

Prepare for this by changing debiantag to debiantag_old and changing
all call sites outside dgit itself.

dgit will want to be able to generate either, depending on whether
it's cooperating in an rpush with a new or old dgit so reintroduce a
wrapper called debiantag there.  This will also allow us to have dgit
generate old tags for testing.

No functional change.

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