t4002 (diff-basic): use test_might_fail for commands that might fail

Also replace '|| return 1' by '&&' to allow chain of operations to be
checked for proper return status, and modify the update-index command
as suggested by Jonathan Nieder to not exit early but try to make sure
files that match the work tree are marked as matching.

Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4002-diff-basic.sh

@@ -205,8 +205,8 @@ test_expect_success \
     'rm -fr Z [A-Z][A-Z] &&
      git read-tree $tree_A &&
      git checkout-index -f -a &&
-     git read-tree --reset $tree_O || return 1
-     git update-index --refresh >/dev/null ;# this can exit non-zero
+     git read-tree --reset $tree_O &&
+     test_must_fail git update-index --refresh -q &&
      git diff-files >.test-a &&
      cmp_diff_files_output .test-a .test-recursive-OA'
 
@@ -215,8 +215,8 @@ test_expect_success \
     'rm -fr Z [A-Z][A-Z] &&
      git read-tree $tree_B &&
      git checkout-index -f -a &&
-     git read-tree --reset $tree_O || return 1
-     git update-index --refresh >/dev/null ;# this can exit non-zero
+     git read-tree --reset $tree_O &&
+     test_must_fail git update-index --refresh -q &&
      git diff-files >.test-a &&
      cmp_diff_files_output .test-a .test-recursive-OB'
 
@@ -225,8 +225,8 @@ test_expect_success \
     'rm -fr Z [A-Z][A-Z] &&
      git read-tree $tree_B &&
      git checkout-index -f -a &&
-     git read-tree --reset $tree_A || return 1
-     git update-index --refresh >/dev/null ;# this can exit non-zero
+     git read-tree --reset $tree_A &&
+     test_must_fail git update-index --refresh -q &&
      git diff-files >.test-a &&
      cmp_diff_files_output .test-a .test-recursive-AB'