Fixing the iteration over all_subcommands in zsh.rs. We deduplicate
values on (sc_name, bin_name) keys, but then only iterate on bin_name.
This doesn't cause problems now, since all bin names seem to be unique.
However, without fixing this, the next commit would have started
generating duplicated functions with same names.
For example, with an #[long = "foo", visible_alias = "bar"] subcommand,
we'll end up with 2 pairs: [("foo", "foo"), ("bar", "foo")]. Before this
commit, we would have ended up generating _my-app__foo_commands()
functions. These functions should have identical content, so it is not
an error, just an inefficiency that we can fix.
