Improve isRepoRoot check to support branch names with slashes
Improve isRepoRoot check to support branch names with slashes

For example: https://github.com/kohana/cache/tree/3.3/master

The branch name `3.3/master` is supported by GitHub as a valid Git branch name.
The only way we could detect such branch names is via a DOM check.

This change also allows dashes in the branch names.

jgierer12/refined-github,sindresorhus/refined-github,busches/refined-github,jgierer12/refined-github,busches/refined-github,sindresorhus/refined-github