blame: allow blame --reverse --first-parent when it makes sense

Allow combining --reverse and --first-parent if initial commit of
specified range is at the first-parent chain starting from the final
commit. Disable the prepare_revision_walk()'s builtin children
collection, instead picking only the ones which are along the first
parent chain.

Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -2502,6 +2502,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	long dashdash_pos, lno;
 	char *final_commit_name = NULL;
 	enum object_type type;
+	struct commit *final_commit = NULL;
 
 	static struct string_list range_list;
 	static int output_option = 0, opt = 0;
@@ -2689,11 +2690,11 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	}
 	else if (contents_from)
 		die("--contents and --children do not blend well.");
-	else if (revs.first_parent_only)
-		die("combining --first-parent and --reverse is not supported");
 	else {
 		final_commit_name = prepare_initial(&sb);
 		sb.commits.compare = compare_commits_by_reverse_commit_date;
+		if (revs.first_parent_only)
+			revs.children.name = NULL;
 	}
 
 	if (!sb.final) {
@@ -2710,6 +2711,14 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	else if (contents_from)
 		die("Cannot use --contents with final commit object name");
 
+	if (reverse && revs.first_parent_only) {
+		struct object_array_entry *entry = find_single_final(sb.revs);
+		if (!entry)
+			die("--reverse and --first-parent together require specified latest commit");
+		else
+			final_commit = (struct commit*) entry->item;
+	}
+
 	/*
 	 * If we have bottom, this will mark the ancestors of the
 	 * bottom commits we would reach while traversing as
@@ -2718,6 +2727,25 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	if (prepare_revision_walk(&revs))
 		die(_("revision walk setup failed"));
 
+	if (reverse && revs.first_parent_only) {
+		struct commit *c = final_commit;
+
+		sb.revs->children.name = "children";
+		while (c->parents &&
+		       hashcmp(c->object.sha1, sb.final->object.sha1)) {
+			struct commit_list *l = xcalloc(1, sizeof(*l));
+
+			l->item = c;
+			if (add_decoration(&sb.revs->children,
+					   &c->parents->item->object, l))
+				die("BUG: not unique item in first-parent chain");
+			c = c->parents->item;
+		}
+
+		if (hashcmp(c->object.sha1, sb.final->object.sha1))
+			die("--reverse --first-parent together require range along first-parent chain");
+	}
+
 	if (is_null_sha1(sb.final->object.sha1)) {
 		o = sb.final->util;
 		sb.final_buf = xmemdupz(o->file.ptr, o->file.size);

t/t8009-blame-vs-topicbranches.sh

@@ -26,7 +26,7 @@ test_expect_success setup '
 	test_merge A3 C1
 	'
 
-test_expect_failure 'blame --reverse --first-parent finds A1' '
+test_expect_success 'blame --reverse --first-parent finds A1' '
 	git blame --porcelain --reverse --first-parent A0..A3 -- file.t >actual_full &&
 	head -n 1 <actual_full | sed -e "s/ .*//" >actual &&
 	git rev-parse A1 >expect &&