redirector: Handing of slashes in branch names seems incorrect

I have just tried deploying from URL:

> https://github.com/ahmetb/golang-samples/tree/run/grpc-server-streaming/run/grpc-server-streaming

where the branch name was `run/grpc-server-streaming`.

funny enough, there's no way for `redirector` to tell whether it is

```
branch = run
dir = grpc-server-streaming/run/grpc-server-streaming
```

or

```
branch = run/grpc-server-streaming
dir =run/grpc-server-streaming
```

So it naturally did some regex matching and wrongly assumed `branch=run`, **despite** specifying ?dir= correctly.

```
cloudshell_open --repo_url "https://github.com/ahmetb/golang-samples.git" --dir "run/grpc-server-streaming" --page "editor" --git_branch "run"
[ ✓ ] Cloned git repository https://github.com/ahmetb/golang-samples.git.
Error: failed to checkout revision "run": git checkout failed: exit status 1, output:
error: pathspec 'run' did not match any file(s) known to git.
```

I don't think we can reliably disambiguate the URL in this case. (GitHub probably does, because it has the data.) Just wanted to note.