[RFE] Add support to find a minimal set of dependent commits to make a commit apply cleanly

Many times a commit fails to apply cleanly due to one or more missing upstream commits. In cases like this, finding a minimal set of missing upstream commit(s) would be more useful than munging the commit to apply with conflicts (differences from the upstream commit). This would not only allow the current commit to apply cleanly, but make future backports simpler by reducing the number of (potential) conflicts.