[boost] [git] Post svn conversion merge point instructions moved

The post svn conversion merge point instructions have been moved to a separate page. See https://svn.boost.org/trac/boost/wiki/PostCvtMergePoint A set of preparatory steps suggested by Peter Dimov have been added. I'm unsure how Peter's prep steps interact with the actual merge point instructions. Particularly, what SHA is used for the merge point? Please take a look, and then suggest corrections or clarifications. Thanks, --Beman