When simplifying to rebase, relax the ff requirement.

When simplifying an incremental merge of BRANCH onto MASTER into a
rebase, a simplification with --branch=BRANCH would fail because
updating BRANCH to the result of the rebase is not a fast-forward
update.

But when doing a rebase, the user typically *wants* to discard the old
versions of the commits on BRANCH (just like with "git rebase").  So
allow such a simplification even if the reference update is not a
fast-forward, *provided* that the apex of the incremental merge (i.e.,
before the simplification) is a descendant of the commit pointed to by
the old value of the reference being pointed at the result.

TODO.rst

@@ -30,11 +30,6 @@ Convenience features
   manual merge conflicts into suggested log messages for the
   simplified commits.
 
-* For "simplify --goal=rebase", relax the requirement that the final
-  result be a fast-forward of its original value.  Instead, require
-  only that *the final incremental merge* commit (i.e., before
-  simplification) be a fast-forward of the original reference.
-
 * Maybe remember the names of the two original branches for use in log
   messages etc.  (Should they be stored locally in ``git config`` or
   shareably in the state blob?)

git-imerge

@@ -2279,6 +2279,23 @@ class MergeState(Block):
                     % (i1, i2)
                     )
 
+        if not force:
+            # A rebase simplification is allowed to discard history,
+            # as long as the *pre-simplification* apex commit is a
+            # descendant of the branch to be moved.
+            try:
+                ref_oldval = get_commit_sha1(refname)
+            except ValueError:
+                # refname doesn't already exist; no problem.
+                pass
+            else:
+                commit = self[-1, -1].sha1
+                if not MergeState._is_ancestor(ref_oldval, commit):
+                    raise Failure(
+                        '%s is not an ancestor of %s; use --force if you are sure'
+                        % (commit, refname,)
+                        )
+
         commit = self[i1, 0].sha1
         for i2 in range(1, self.len2):
             orig = self[0, i2].sha1
@@ -2290,7 +2307,9 @@ class MergeState(Block):
                 tree, [commit], msg=get_log_message(orig), metadata=authordata,
                 )
 
-        self._set_refname(refname, commit, force=force)
+        # We checked above that the update is OK, so here we can set
+        # force=True:
+        self._set_refname(refname, commit, force=True)
 
     def simplify_to_merge(self, refname, force=False):
         if not (-1, -1) in self: