Support "/" character in branch names

[mtojek]

Having / in Git branch name is legal, but cmux does not accept it. I suspect this is because it might be interpreted as a system path.

cmux now (Workspace name can only contain lowercase letters, digits, underscore, and hyphen):

[ammario]

It's challenging that we use the name in file paths, a feature which I quite enjoy.

I suppose we can replace "/" with "-" for the purpose of paths and then fail if their are conflicts.