don't push before creating a pull (#334)

Not sure if this is the best way, but it's the simplest way to fix #333.
Everything else is overly complex due to a chicken-egg problem:
Knowing which remote / branch to push involves requires prompting the user,
which requires to have a upstream branch pushed to detect default values.

Co-authored-by: Norwin Roosen <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/334
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Reviewed-by: khmarbaise <khmarbaise@noreply.gitea.io>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

modules/git/branch.go

@@ -223,5 +223,5 @@ func (r TeaRepo) TeaGetCurrentBranchName() (string, error) {
 		return "", fmt.Errorf("active ref is no branch")
 	}
 
-	return strings.TrimPrefix(localHead.Name().String(), "refs/heads/"), nil
+	return localHead.Name().Short(), nil
 }