subtree: improve decision on merges kept in split

When multiple identical parents are detected for a commit being considered for copying, explicitly check whether one is the common merge base between the commits. If so, the other commit can be used as the identical parent; if not, a merge must be performed to maintain history. In some situations two parents of a merge commit may appear to both have identical subtree content with each other and the current commit. However, those parents can potentially come from different commit graphs. Previous behavior would simply select one of the identical parents to serve as the replacement for this commit, based on the order in which they were processed. New behavior compares the merge base between the commits to determine if a new merge commit is necessary to maintain history despite the identical content. Signed-off-by: Strain, Roger L <roger.strain@swri.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>