This is the function which does the work for Repository.merge_commits()
and allows us more direct control over which trees we want to merge,
including trees which do not belong to commits.
I've gone with taking a string and converting it because the depth of
the namespacing in the libgit2 name is rather large and we don't care
about the global namespace that C has; and this also lets us pass the
same as the '-X' option to git-merge.
