dgit: factor out build_or_push_prep_early sub

dgit: factor out build_or_push_prep_early sub

This is to permit code deduplication between the commands for pushing
and building.  Both these sets of commands deny usage of -p, and set
$isuite, $package and $version based on the changelog.

In particular, a command that will perform a build /and/ a push can
first perform the check for -p, set $package and then call functions
to do the build.  These latter functions will not complain that
$package is set thanks to the $build_or_push_prep_early_done logic.

No functional change.

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