WIP: split DROPPED part to its own commit! rebase -i -p: use patch-id directly to determine the dropped commits
We need to find out which commits were dropped by --cherry-pick, and
which commits they correspond to.
As before, we substitute the dropped commits by their parents.
If the dropped commit has no parent, it is the root commit, whose
new parent is the commit we are rebasing onto.
While at it, we record the information in "$REWRITTEN" instead of
"$DROPPED", as there is no semantical difference in their usage.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>