mq: refactor patchheader header ordering to match export (BC) The refactoring also gives more robust and extendable handling of other HG headers.

mq: upgrade non-plain patches to HG format when setting parent in patchheader Parent will now always be updated or added when qrefreshing HG patches. Plain patches will not be changed, but patches that neither are plain nor HG will be upgraded to HG patches on first refresh.

mq: make patchheader .plainmode more reliable Instead of having to make extra checks whenever we use .plainmode, let the initial value consider the actual patch header content.