Skip validation when pushing new branches (#16064)

The Puppet code validation Git hook was exiting with an error when
receiving new branches because in those cases $oldrev equals 0{40}.

I experimented with a solution grepping the output of:

    git ls-tree --full-tree -r --name-only HEAD

But I thought the output was too extensive so for now I just skip
validation when pushing a new branch. We will always eventually hit the
pre-receive hook when pushing to master, anyway.