RecursiveMerger: do not insert virtual commits into repository
When merging common ancestors to create a single virtual common
ancestor the commit does not need to be inserted into the Git
repository. Instead just mock it out in memory as part of the
merger's RevWalk pool.
Make the author and committer stable and predictable for any
given pair of merge bases. It is not necessary for the caller's
name or email to be used as the commit will not be written out.