Fix t6031 on filesystems without working exec bit

The point of the test is not really to test the ability of the
filesystem to keep the given x-bit, but to check is merge-recursive
correctly handles it.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6031-merge-recursive.sh

@@ -3,6 +3,9 @@
 test_description='merge-recursive: handle file mode'
 . ./test-lib.sh
 
+# Note that we follow "chmod +x F" with "update-index --chmod=+x F" to
+# help filesystems that do not have the executable bit.
+
 test_expect_success 'mode change in one branch: keep changed version' '
 	: >file1 &&
 	git add file1 &&
@@ -13,7 +16,7 @@ test_expect_success 'mode change in one branch: keep changed version' '
 	git commit -m a &&
 	git checkout -b b1 master &&
 	chmod +x file1 &&
-	git add file1 &&
+	git update-index --chmod=+x file1 &&
 	git commit -m b1 &&
 	git checkout a1 &&
 	git merge-recursive master -- a1 b1 &&
@@ -26,7 +29,7 @@ test_expect_success 'mode change in both branches: expect conflict' '
 	: >file2 &&
 	H=$(git hash-object file2) &&
 	chmod +x file2 &&
-	git add file2 &&
+	git update-index --add --chmod=+x file2 &&
 	git commit -m a2 &&
 	git checkout -b b2 master &&
 	: >file2 &&