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.

bundle2: remove heads and common arguments to getbundle parts generators

bundle2: separate bundle10 and bundle2 cases in getbundle() The primary goal is to make it easier for extensions to alter how bundle2 parts are laid out. They now can use the getbundle2partsgenerator decorator to add new parts, or directly act on getbundle2partsmapping to wrap existing part functions. Note the 'request for bundle10 must include changegroup' error was kept under the same conditions as before, although the logic changes don't make it obvious.

bundle2: pass b2caps down to functions adding bundle2 parts for getbundle