revert: plug memory leak in "cherry-pick root commit" codepath

The empty tree passed as common ancestor to merge_trees() when
cherry-picking a parentless commit is allocated on the heap and never
freed.  Leaking such a small one-time allocation is not a very big
problem, but now that "git cherry-pick" can cherry-pick multiple
commits it can start to add up.

Avoid the leak by storing the fake tree exactly once in the BSS
section (i.e., use a static).  While at it, let's add a test to make
sure cherry-picking multiple parentless commits continues to work.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/revert.c

@@ -273,12 +273,7 @@ static void write_message(struct strbuf *msgbuf, const char *filename)
 
 static struct tree *empty_tree(void)
 {
-	struct tree *tree = xcalloc(1, sizeof(struct tree));
-
-	tree->object.parsed = 1;
-	tree->object.type = OBJ_TREE;
-	pretend_sha1_file(NULL, 0, OBJ_TREE, tree->object.sha1);
-	return tree;
+	return lookup_tree((const unsigned char *)EMPTY_TREE_SHA1_BIN);
 }
 
 static NORETURN void die_dirty_index(const char *me)

t/t3503-cherry-pick-root.sh

@@ -16,15 +16,40 @@ test_expect_success setup '
 	echo second > file2 &&
 	git add file2 &&
 	test_tick &&
-	git commit -m "second"
+	git commit -m "second" &&
+
+	git symbolic-ref HEAD refs/heads/third &&
+	rm .git/index file2 &&
+	echo third > file3 &&
+	git add file3 &&
+	test_tick &&
+	git commit -m "third"
 
 '
 
 test_expect_success 'cherry-pick a root commit' '
 
+	git checkout second^0 &&
 	git cherry-pick master &&
 	test first = $(cat file1)
 
 '
 
+test_expect_success 'cherry-pick two root commits' '
+
+	echo first >expect.file1 &&
+	echo second >expect.file2 &&
+	echo third >expect.file3 &&
+
+	git checkout second^0 &&
+	git cherry-pick master third &&
+
+	test_cmp expect.file1 file1 &&
+	test_cmp expect.file2 file2 &&
+	test_cmp expect.file3 file3 &&
+	git rev-parse --verify HEAD^^ &&
+	test_must_fail git rev-parse --verify HEAD^^^
+
+'
+
 test_done