commands.import: accept a prefix option The --prefix option is meant to be relative to the root rather than the current working directory. This is for consistency with the rest of 'hg import' -- paths in patches are otherwise considered to be relative to the root. In upcoming patches we'll hook this option up to the patch functions. --exact with --prefix is currently disallowed because I can't really come up with sensible semantics for it, especially when only part of the patch is preserved.