util: concentrate quoting knowledge to windows.py quotecommand()
This fixes all callers of util.quotecommand() and place special knowledge
of the bugfix in 2.7.1 in a single platform specific place.
tests: clean up test-rollback.t
The last comment is on the last file line and is useless, seems
like it was introduced in
8fdc11fec6ae during unification.