libgit2/rugged — pull request #168 by spraints (2013-03-14 19:40:12+00:00)
Title: Baseless merge base

So, I missed [this](http://libgit2.github.com/libgit2/#HEAD/group/merge/git_merge_base) on the first try:

> returns
> int - Zero on success; **GIT_ENOTFOUND** or -1 on failure.

This fixes the problem.