Fix make output: run 'git push' before 'gbp push'

As mentiond in Debian#107, running 'gbp push' when there's no debian tag is
not the right thing to do,  as it will only push the upstream branch.
Consequences are:

1. The default gitlab branch is set to the upstream branch instead of
   the debian branch.
2. The debian branch is not pushed and needs to be pushed manually
   anyway.

I believe that the best thing to do instead is just to run 'git push
origin <debian-branch>' first, and then run 'gbp push'.

Closes: Debian#107

Author: elboulangero

make.go

@@ -1000,6 +1000,7 @@ func execMake(args []string, usage func()) {
 	fmt.Printf("    dh-make-golang create-salsa-project %s\n", debsrc)
 	fmt.Printf("\n")
 	fmt.Printf("Once you are happy with your packaging, push it to salsa using:\n")
+	fmt.Printf("    git push origin %s\n", debBranch)
 	fmt.Printf("    gbp push\n")
 	fmt.Printf("\n")