replay: support replaying 2-parent merges

Documentation/git-history.adoc

@@ -40,13 +40,26 @@ at once.
 LIMITATIONS
 -----------
 
-This command does not (yet) work with histories that contain merges. You
-should use linkgit:git-rebase[1] with the `--rebase-merges` flag instead.
-
-Furthermore, the command does not support operations that can result in merge
-conflicts. This limitation is by design as history rewrites are not intended to
-be stateful operations. The limitation can be lifted once (if) Git learns about
-first-class conflicts.
+This command supports two-parent merge commits in the rewrite path:
+the auto-remerged tree of the original parents, the merge commit
+itself, and the auto-merged tree of the rewritten parents are
+combined so that the user's manual conflict resolution (textual or
+semantic) is preserved through the replay. Octopus merges (more than
+two parents) are not supported and are rejected with an error.
+
+The replay propagates the textual diffs the user actually made in
+the merge commit. It does _not_ extrapolate symbol-level intent: if
+rewriting the parents pulls in genuinely new content (for example, a
+new caller of a function that the merge renamed), that new content
+is _not_ rewritten by the replay and may need a follow-up edit.
+Symbol-aware refactoring is out of scope here, just as it is for
+plain rebase.
+
+The command does not support operations that can result in merge
+conflicts on the replayed merge itself. This limitation is by design
+as history rewrites are not intended to be stateful operations. Use
+linkgit:git-rebase[1] with the `--rebase-merges` flag when the
+rewrite is expected to require interactive conflict resolution.
 
 COMMANDS
 --------

Makefile

@@ -832,6 +832,7 @@ TEST_BUILTINS_OBJS += test-hash-speed.o
 TEST_BUILTINS_OBJS += test-hash.o
 TEST_BUILTINS_OBJS += test-hashmap.o
 TEST_BUILTINS_OBJS += test-hexdump.o
+TEST_BUILTINS_OBJS += test-historian.o
 TEST_BUILTINS_OBJS += test-json-writer.o
 TEST_BUILTINS_OBJS += test-lazy-init-name-hash.o
 TEST_BUILTINS_OBJS += test-match-trees.o

builtin/history.c

@@ -195,15 +195,15 @@ static int parse_ref_action(const struct option *opt, const char *value, int uns
 	return 0;
 }
 
-static int revwalk_contains_merges(struct repository *repo,
-				   const struct strvec *revwalk_args)
+static int revwalk_contains_octopus_merges(struct repository *repo,
+					   const struct strvec *revwalk_args)
 {
 	struct strvec args = STRVEC_INIT;
 	struct rev_info revs;
 	int ret;
 
 	strvec_pushv(&args, revwalk_args->v);
-	strvec_push(&args, "--min-parents=2");
+	strvec_push(&args, "--min-parents=3");
 
 	repo_init_revisions(repo, &revs, NULL);
 
@@ -217,7 +217,7 @@ static int revwalk_contains_merges(struct repository *repo,
 	}
 
 	if (get_revision(&revs)) {
-		ret = error(_("replaying merge commits is not supported yet!"));
+		ret = error(_("replaying octopus merges is not supported"));
 		goto out;
 	}
 
@@ -289,7 +289,7 @@ static int setup_revwalk(struct repository *repo,
 		strvec_push(&args, "HEAD");
 	}
 
-	ret = revwalk_contains_merges(repo, &args);
+	ret = revwalk_contains_octopus_merges(repo, &args);
 	if (ret < 0)
 		goto out;
 
@@ -482,6 +482,9 @@ static int cmd_history_reword(int argc,
 	if (ret < 0) {
 		ret = error(_("failed replaying descendants"));
 		goto out;
+	} else if (ret) {
+		ret = error(_("conflict during replay; some descendants were not rewritten"));
+		goto out;
 	}
 
 	ret = 0;
@@ -721,6 +724,9 @@ static int cmd_history_split(int argc,
 	if (ret < 0) {
 		ret = error(_("failed replaying descendants"));
 		goto out;
+	} else if (ret) {
+		ret = error(_("conflict during replay; some descendants were not rewritten"));
+		goto out;
 	}
 
 	ret = 0;