Allow scripts/repo to use remote branches other than default

[mononaut]

It's useful to be able to checkout specific branches of a development fork, to switch between different versions of an app for testing and troubleshooting.

Before the v0.5 release, you could do this via scripts/update/update --repo <user>/<repo>#<branch>, but the new "repo" script for umbrel-apps seems to be limited to the default branch of the given repository.

This PR adds an extra optional argument to that script to specify a branch.

e.g.

./scripts/repo set https://github.com/mononaut/umbrel-apps.git bitfeed-v2.3.3
./scripts/repo update

If the branch argument is omitted, the script uses the default branch of the repo as before.