am: avoid directory rename detection when calling recursive merge machinery

Let's say you have the following three trees, where Base is from one commit
behind either master or branch:

   Base  : bar_v1, foo/{file1, file2, file3}
   branch: bar_v2, foo/{file1, file2},       goo/file3
   master: bar_v3, foo/{file1, file2, file3}

Using git-am (or am-based rebase) to apply the changes from branch onto
master results in the following tree:

   Result: bar_merged, goo/{file1, file2, file3}

This is not what users want; they did not rename foo/ -> goo/, they only
renamed one file within that directory.  The reason this happens is am
constructs fake trees (via build_fake_ancestor()) of the following form:

   Base_bfa  : bar_v1, foo/file3
   branch_bfa: bar_v2, goo/file3

Combining these two trees with master's tree:

   master: bar_v3, foo/{file1, file2, file3},

You can see that merge_recursive_generic() would see branch_bfa as renaming
foo/ -> goo/, and master as just adding both foo/file1 and foo/file2.  As
such, it ends up with goo/{file1, file2, file3}

The core problem is that am does not have access to the original trees; it
can only construct trees using the blobs involved in the patch.  As such,
it is not safe to perform directory rename detection within am -3.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1596,6 +1596,7 @@ static int fall_back_threeway(const struct am_state *state, const char *index_pa
 	o.branch1 = "HEAD";
 	their_tree_name = xstrfmt("%.*s", linelen(state->msg), state->msg);
 	o.branch2 = their_tree_name;
+	o.detect_directory_renames = 0;
 
 	if (state->quiet)
 		o.verbosity = 0;

t/t3401-rebase-and-am-rename.sh

@@ -152,7 +152,7 @@ test_expect_success 'rebase --interactive: NO directory rename' '
 	)
 '
 
-test_expect_failure 'rebase (am): NO directory rename' '
+test_expect_success 'rebase (am): NO directory rename' '
 	test_when_finished "git -C no-dir-rename rebase --abort" &&
 	(
 		cd no-dir-rename &&
@@ -190,7 +190,7 @@ test_expect_success 'rebase --merge: NO directory rename' '
 	)
 '
 
-test_expect_failure 'am: NO directory rename' '
+test_expect_success 'am: NO directory rename' '
 	test_when_finished "git -C no-dir-rename am --abort" &&
 	(
 		cd no-dir-rename &&