magit-branch: new command, adjust popup
drop `dirty' submodule section type

Just use `diff' for now, like for submodules with committed changes.
While a `dirty' (only) submodule cannot be staged many other commands
are valid, and these have not yet been defined for the `dirty' type.
magit-describe-section: show value again
magit-wash-commit: don't call magit-wash-diffstats directly

We also call `magit-wash-diffs' here and that already calls the latter.