Use annotated commits to act as our virtual bases, instead of regular
commits, to avoid polluting the odb with virtual base commits and
trees.  Instead, build an annotated commit with an index and pointers
to the commits that it was merged from.

When there are more than two common ancestors, continue merging the
virtual base with the additional common ancestors, effectively
octopus merging a new virtual base.

This extra constructor will be useful for the annotated versions of
ref-modifying functions, as it allows us to create a commit with the
extended sha syntax which was used to retrieve it.

Rename git_merge_head to git_annotated_commit, as it becomes used
in more operations than just merge.