replay: drop commits that become empty

If the changes in a commit being replayed are already in the branch
that the commits are being replayed onto then drop the commit instead
of creating an empty commit. This matches the behavior of "git rebase
--reapply-cherry-pick --empty=drop" and "git cherry-pick --empty-drop".

Author: phillipwood

replay.c

@@ -4,6 +4,7 @@
 #include "commit.h"
 #include "environment.h"
 #include "gettext.h"
+#include "hex.h"
 #include "ident.h"
 #include "object.h"
 #include "object-name.h"
@@ -88,12 +89,12 @@ struct commit *replay_pick_regular_commit(struct repository *repo,
 					  struct merge_result *result)
 {
 	struct commit *base, *replayed_base;
-	struct tree *pickme_tree, *base_tree;
+	struct tree *pickme_tree, *base_tree, *replayed_base_tree;
 
 	base = pickme->parents->item;
 	replayed_base = mapped_commit(replayed_commits, base, onto);
 
-	result->tree = repo_get_commit_tree(repo, replayed_base);
+	replayed_base_tree = repo_get_commit_tree(repo, replayed_base);
 	pickme_tree = repo_get_commit_tree(repo, pickme);
 	base_tree = repo_get_commit_tree(repo, base);
 
@@ -103,13 +104,17 @@ struct commit *replay_pick_regular_commit(struct repository *repo,
 
 	merge_incore_nonrecursive(merge_opt,
 				  base_tree,
-				  result->tree,
+				  replayed_base_tree,
 				  pickme_tree,
 				  result);
 
 	free((char*)merge_opt->ancestor);
 	merge_opt->ancestor = NULL;
 	if (!result->clean)
 		return NULL;
+	/* Drop commits that become empty */
+	if (oideq(&replayed_base_tree->object.oid, &result->tree->object.oid) &&
+	    !oideq(&pickme_tree->object.oid, &base_tree->object.oid))
+		return replayed_base;
 	return replay_create_commit(repo, result->tree, pickme, replayed_base);
 }

t/t3650-replay-basics.sh

@@ -25,6 +25,8 @@ test_expect_success 'setup' '
 	git switch -c topic3 &&
 	test_commit G &&
 	test_commit H &&
+	git switch -c empty &&
+	git commit --allow-empty --only -m empty &&
 	git switch -c topic4 main &&
 	test_commit I &&
 	test_commit J &&
@@ -106,6 +108,28 @@ test_expect_success 'using replay on bare repo to perform basic cherry-pick' '
 	test_cmp expect result-bare
 '
 
+test_expect_success 'commits that become empty are dropped' '
+	git replay --ref-action=print --advance main topic1^! >result &&
+	ONTO=$(cut -f 3 -d " " result) &&
+	git replay --ref-action=print --onto $ONTO \
+		--branches --ancestry-path=empty ^A >result &&
+	# Write the new value of refs/heads/empty to "new-empty" and
+	# generate a sed script that annotates the output of
+	# `git log --format="%H %s"` with the updated branches
+	SCRIPT="$(sed -e "
+		/empty/{
+			h
+			s|^.*empty \([^ ]*\) .*|\1|wnew-empty
+			g
+		}
+		s|^.*/\([^/ ]*\) \([^ ]*\).*|/\2/s/\\\$/ (\1)/|
+		\$s|\$|;s/^[^ ]* //|" result)" &&
+	git log --format="%H %s" --stdin <new-empty >actual.raw &&
+	sed -e "$SCRIPT" actual.raw >actual &&
+	test_write_lines "empty (empty)" "H (topic3)" G "C (topic1)" F M L B A >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'replay on bare repo fails with both --advance and --onto' '
 	test_must_fail git -C bare replay --advance main --onto main topic1..topic2 >result-bare
 '