No merge is possible. (Unused.)

A "normal" merge; both HEAD and the given merge input have diverged from their common ancestor. The divergent commits must be merged.

All given merge inputs are reachable from HEAD, meaning the repository is up-to-date and no merge needs to be performed.

The given merge input is a fast-forward from HEAD and no merge needs to be performed. Instead, the client can check out the given merge input.

The HEAD of the current repository is "unborn" and does not point to a valid commit. No merge can be performed, but the caller may wish to simply set HEAD to the target commit(s).