merge-tree: fix where two branches share no changes

15b4f7a (merge-tree: use ll_merge() not xdl_merge(), 2010-01-16)
introduced a regression to merge-tree to cause it to segfault when merging
files which existed in one branch, but not in the other or in the
merge-base. This was caused by referencing entry->path at a time when
entry was known to be possibly-NULL.

To correct the problem, we save the path of the entry we came in with,
as the path should be the same among all the stages no matter which
sides are involved in the merge.

Signed-off-by: Will Palmer <wmpalmer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-tree.c

@@ -60,6 +60,7 @@ static void *result(struct merge_list *entry, unsigned long *size)
 {
 	enum object_type type;
 	struct blob *base, *our, *their;
+	const char *path = entry->path;
 
 	if (!entry->stage)
 		return read_sha1_file(entry->blob->object.sha1, &type, size);
@@ -76,7 +77,7 @@ static void *result(struct merge_list *entry, unsigned long *size)
 	their = NULL;
 	if (entry)
 		their = entry->blob;
-	return merge_file(entry->path, base, our, their, size);
+	return merge_file(path, base, our, their, size);
 }
 
 static void *origin(struct merge_list *entry, unsigned long *size)

t/t4300-merge-tree.sh

@@ -24,7 +24,7 @@ EXPECTED
 	test_cmp expected actual
 '
 
-test_expect_failure 'file add !A, B' '
+test_expect_success 'file add !A, B' '
 	cat >expected <<\EXPECTED &&
 added in local
   our    100644 43d5a8ed6ef6c00ff775008633f95787d088285d ONE
@@ -195,7 +195,7 @@ EXPECTED
 	test_cmp expected actual
 '
 
-test_expect_failure 'file remove !A, B' '
+test_expect_success 'file remove !A, B' '
 	cat >expected <<\EXPECTED &&
 removed in remote
   base   100644 43d5a8ed6ef6c00ff775008633f95787d088285d ONE
@@ -213,7 +213,7 @@ EXPECTED
 	test_cmp expected actual
 '
 
-test_expect_failure 'file change A, remove B' '
+test_expect_success 'file change A, remove B' '
 	cat >expected <<\EXPECTED &&
 removed in remote
   base   100644 43d5a8ed6ef6c00ff775008633f95787d088285d ONE