Assorted contrib/subtree Patches

All of the patches I have received from others as well as a few of my own follow. Probably the most controversial is a patch to remove --annotate. After some discussion on the list it became clear that we really want a more general commit rewrite feature. Removing --annotate means we don't have to also support --unannotate and carry both forward as backward-compatibility baggage.

Before --annotate was added, git-subtree would force an annotation of "*" on every split commit message. It now does no such thing so there's no need to unannotate anything.

Please review and integrate.  Thanks!

                        -David