subtree: use commits before rejoins for splits

Adds recursive evaluation of parent commits which were not part of the initial commit list when performing a split. Split expects all relevant commits to be reachable from the target commit but not reachable from any previous rejoins. However, a branch could be based on a commit prior to a rejoin, then later merged back into the current code. In this case, a parent to the commit will not be present in the initial list of commits, trigging an "incorrect order" warning. Previous behavior was to consider that commit to have no parent, creating an original commit containing all subtree content. This commit is not present in an existing subtree commit graph, changing commit hashes and making pushing to a subtree repo impossible. New behavior will recursively check these unexpected parent commits to track them back to either an earlier rejoin, or a true original commit. The generated synthetic commits will properly match previously-generated commits, allowing successful pushing to a prior subtree repo. Signed-off-by: Strain, Roger L <roger.strain@swri.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>