rebase -i --keep-empty: don't prune empty commits

If there are empty commits on the left hand side of $upstream...HEAD
then the empty commits on the right hand side that we want to keep are
pruned by --cherry-pick. Fix this by using --cherry-mark instead of
--cherry-pick and keeping the commits that are empty or are not marked
as cherry-picks.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -2470,7 +2470,7 @@ int sequencer_make_script(FILE *out, int argc, const char **argv,
 	init_revisions(&revs, NULL);
 	revs.verbose_header = 1;
 	revs.max_parents = 1;
-	revs.cherry_pick = 1;
+	revs.cherry_mark = 1;
 	revs.limited = 1;
 	revs.reverse = 1;
 	revs.right_only = 1;
@@ -2495,8 +2495,12 @@ int sequencer_make_script(FILE *out, int argc, const char **argv,
 		return error(_("make_script: error preparing revisions"));
 
 	while ((commit = get_revision(&revs))) {
+		int is_empty  = is_original_commit_empty(commit);
+
+		if (!is_empty && (commit->object.flags & PATCHSAME))
+			continue;
 		strbuf_reset(&buf);
-		if (!keep_empty && is_original_commit_empty(commit))
+		if (!keep_empty && is_empty)
 			strbuf_addf(&buf, "%c ", comment_line_char);
 		strbuf_addf(&buf, "%s %s ", insn,
 			    oid_to_hex(&commit->object.oid));

t/t3421-rebase-topology-linear.sh

@@ -215,7 +215,7 @@ test_run_rebase () {
 }
 test_run_rebase success ''
 test_run_rebase failure -m
-test_run_rebase failure -i
+test_run_rebase success -i
 test_run_rebase failure -p
 
 #       m