mdiff.diffopts: add a new noprefix option
By popular demand, we introduce an option to disable the 'a/' and 'b/'
prefixes in diff output. This makes copying and pasting filenames from diff
output easier.
This option will be implemented and documented in upcoming patches. To ensure
that existing scripts that parse output don't break, we will ensure that this
prefix is disabled in plain mode. A straight 'hg export | hg import' without
HGPLAIN=1 will still be broken though, but there's little that can be done
about that.