Remote subtree split --annotate

This is a fairly simple patch to remove subtree's split --annotate option. The intent of the option is to mark commits split from a subdirectory but git has other tools more suitable for editing commit messages. Given --annotate an --unannotate feature seems obvious and in fact was requested at one point. Discussion ensued and it became clear that the definition of --unannotate is not obvious given the multitude of ways one might want to rewrite a commit message.

Since it is simple to run other git commit message rewriting tools on commits split from a subdirectory, it seems best to just get git-subtree out of the commit rewriting business altogether.