Introduce git_merge_head_id

Callers (especially bindings) may want to know if they can fast-forward, by creating a `git_merge_head` and then inquiring about the merge status.  Having a way to get the ID back out of the merge head is useful in the case where you want to perform that fast-forward.  Otherwise, users would have to lug around both the merge head and the ID that it was created from.