Added git_treebuilder_entrycount

I need this method in order to implement a significant optimization in hlibgit2, since it frees me entirely from having to deal with git_tree objects, and I can work solely with treebuilders to generate tree oids.

It would also be very nice to have a version of commit which, instead of requiring a git_tree and an array of git_commits for parents, would access just git_oid values for both, since I've already written all the information to disk. In fact, I'll put this next on my list!