This patch adds an optional MergePolicy field to CommitOptions. It's a
callback. If the caller sets it, then the commit code will look for a
common ancestor between the Dataset HEAD and the provided Commit. If
the caller-provided Commit descends from HEAD, then Commit proceeds as
normal.
If it does not, but there is a common ancestor, the code runs
merge.ThreeWay() on the values of the provided Commit, HEAD, and the
common ancestor, invoking the MergePolicy callback to resolve
conflicts. If merge succeeds, a merge Commit is created that descends
from both HEAD and the caller-provided Commit. This becomes the new
HEAD of the Dataset.
Fixes#2534
Noms SDK users frequently shoot themselves in the foot because they're
holding onto an "old" Database object. That is, they have a Database
tucked away in some internal state, they call Commit() on it, and
don't replace the object in their internal state with the new Database
returned from Commit.
This PR changes the Database and Dataset Go API to be in line with the
proposal in Issue #2589. JS follows in a separate patch.
