Split tags: Preparation: Rename tag variables etc. in dopush

Split tags: Preparation: Rename tag variables etc. in dopush

We are going to have different branches at once.  Rename some
variables to permit this distinction.

We have $actualhead (whatever HEAD is), $dgithead (the dgit view), and
$maintviewhead (the maintainer's view if different, or undef if not0.

Change the existing references.  Also change one reference to HEAD to
$dgithead: when we update the local tracking ref for the dgit view, we
need to use $dgitview.

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