git-debpush: use -m rather than -F- <<EOF

git-debpush: use -m rather than -F- <<EOF

This means that a run with set -x shows the tag data that is being
signed.  The user can even then try it again.  It will make it easier
to do some kind of dry run, too, if we want to.

We are really not in danger of hitting a command line length limit.

(It also involves fewer syscalls, less use of /tmp etc., although that
is really irrelevant here.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Acked-by: Sean Whitton <spwhitton@spwhitton.name>