case "$a" in
                        START) sq="$b" ;;
                        git-subtree-mainline:) main="$b" ;;
-                       git-subtree-split:) sub="$b" ;;
+                       git-subtree-split:)
+                               sub="$(git rev-parse "$b^0")" ||
+                                   die "could not rev-parse split hash $b from commit $sq"
+                               ;;
                        END)
                                if [ -n "$sub" ]; then
                                        if [ -n "$main" ]; then
                case "$a" in
                        START) sq="$b" ;;
                        git-subtree-mainline:) main="$b" ;;
-                       git-subtree-split:) sub="$b" ;;
+                       git-subtree-split:)
+                               sub="$(git rev-parse "$b^0")" ||
+                                   die "could not rev-parse split hash $b from commit $sq"
+                               ;;
                        END)
                                debug "  Main is: '$main'"
                                if [ -z "$main" -a -n "$sub" ]; then