tests: do not use implicit "git diff --no-index"

As a general principle, we should not use "git diff" to validate the
results of what git command that is being tested has done.  We would not
know if we are testing the command in question, or locating a bug in the
cute hack of "git diff --no-index".

Rather use test_cmp for that purpose.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3040-subprojects-basic.sh

@@ -24,7 +24,7 @@ test_expect_success 'create subprojects' \
     git add sub2 &&
     git commit -q -m "subprojects added" &&
     git diff-tree --abbrev=5 HEAD^ HEAD |cut -d" " -f-3,5- >current &&
-    git diff expected current'
+    test_cmp expected current'
 
 git branch save HEAD
 
@@ -62,7 +62,7 @@ test_expect_success 'check if clone works' \
     'git ls-files -s >expected &&
     git clone -l -s . cloned &&
     ( cd cloned && git ls-files -s ) >current &&
-    git diff expected current'
+    test_cmp expected current'
 
 test_expect_success 'removing and adding subproject' \
     'git update-index --force-remove -- sub2 &&