CommitCommand already provided a method to set the comment which should
be written into the reflog. The underlying RefUpdate class supported to
skip writing a reflog entry. But through the CommitCommand API it was
not possible to prevent writing a reflog entry. Fix this and allow
creating commits which don't occur in the reflog.
Change-Id: I193c53de71fb5958ea749c4bfa8360a51acc9b58