Remove #{git_clean} / #{git_flags} should expand to ✔ if working tree is clean

[arl]

As it was also the case in bash-git-prompt, the ✔ symbol, indicating a clean working tree, should not be a separate keyword.

Instead #{git_flags} should expand to ✔ when the working tree is clean, and to the succession of dirty flags otherwise. They are mutually exclusive and they both represent related information.

This would also make simpler the status definition.