Tests: use test_cmp instead of diff where possible

Several old tests were written before test_cmp was introduced, convert
these to test_cmp.

If were are at it, fix the order of the arguments where necessary to
make expected come first, so the command shows how the test result
deviates from the correct output.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -127,7 +127,7 @@ cat >expected <<\EOF
 EOF
 test_expect_success \
     'validate git ls-files output for a known tree.' \
-    'diff current expected'
+    'test_cmp expected current'
 
 test_expect_success \
     'writing tree out with git write-tree.' \
@@ -147,7 +147,7 @@ cat >expected <<\EOF
 EOF
 test_expect_success \
     'git ls-tree output for a known tree.' \
-    'diff current expected'
+    'test_cmp expected current'
 
 # This changed in ls-tree pathspec change -- recursive does
 # not show tree nodes anymore.
@@ -166,7 +166,7 @@ cat >expected <<\EOF
 EOF
 test_expect_success \
     'git ls-tree -r output for a known tree.' \
-    'diff current expected'
+    'test_cmp expected current'
 
 # But with -r -t we can have both.
 test_expect_success \
@@ -187,7 +187,7 @@ cat >expected <<\EOF
 EOF
 test_expect_success \
     'git ls-tree -r output for a known tree.' \
-    'diff current expected'
+    'test_cmp expected current'
 
 test_expect_success \
     'writing partial tree out with git write-tree --prefix.' \