dgit-maint-merge(7): Mandate pushing upstream branch to alioth

In the case where we are using gbp-import-orig(1) to maintain a
virtual upstream branch, we must push that branch somewhere.  It is
needed whenever we import a new upstream release.

Thanks to Johannes Schauer for pointing out that the manpage
previously said that pushing to alioth is always optional.

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>