tests: use 'test_must_be_empty' instead of 'test_cmp <empty> <out>'

Using 'test_must_be_empty' is shorter and more idiomatic than

  >empty &&
  test_cmp empty out

as it saves the creation of an empty file.  Furthermore, sometimes the
expected empty file doesn't have such a descriptive name like 'empty',
and its creation is far away from the place where it's finally used
for comparison (e.g. in 't7600-merge.sh', where two expected empty
files are created in the 'setup' test, but are used only about 500
lines later).

These cases were found by instrumenting 'test_cmp' to error out the
test script when it's used to compare empty files, and then converted
manually.

Note that even after this patch there still remain a lot of cases
where we use 'test_cmp' to check empty files:

  - Sometimes the expected output is not hard-coded in the test, but
    'test_cmp' is used to ensure that two similar git commands produce
    the same output, and that output happens to be empty, e.g. the
    test 'submodule update --merge  - ignores --merge  for new
    submodules' in 't7406-submodule-update.sh'.

  - Repetitive common tasks, including preparing the expected results
    and running 'test_cmp', are often extracted into a helper
    function, and some of this helper's callsites expect no output.

  - For the same reason as above, the whole 'test_expect_success'
    block is within a helper function, e.g. in 't3070-wildmatch.sh'.

  - Or 'test_cmp' is invoked in a loop, e.g. the test 'cvs update
    (-p)' in 't9400-git-cvsserver-server.sh'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7400-submodule-basic.sh

@@ -101,7 +101,6 @@ inspect() {
 
 test_expect_success 'submodule add' '
 	echo "refs/heads/master" >expect &&
-	>empty &&
 
 	(
 		cd addtest &&
@@ -123,7 +122,7 @@ test_expect_success 'submodule add' '
 	inspect addtest/submod ../.. &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success 'setup parent and one repository' '
@@ -188,7 +187,6 @@ test_expect_success 'submodule add --branch' '
 	refs/heads/initial
 	refs/heads/master
 	EOF
-	>empty &&
 
 	(
 		cd addtest &&
@@ -201,12 +199,11 @@ test_expect_success 'submodule add --branch' '
 	inspect addtest/submod-branch ../.. &&
 	test_cmp expect-heads heads &&
 	test_cmp expect-head head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success 'submodule add with ./ in path' '
 	echo "refs/heads/master" >expect &&
-	>empty &&
 
 	(
 		cd addtest &&
@@ -218,12 +215,11 @@ test_expect_success 'submodule add with ./ in path' '
 	inspect addtest/dotsubmod/frotz ../../.. &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success 'submodule add with /././ in path' '
 	echo "refs/heads/master" >expect &&
-	>empty &&
 
 	(
 		cd addtest &&
@@ -235,12 +231,11 @@ test_expect_success 'submodule add with /././ in path' '
 	inspect addtest/dotslashdotsubmod/frotz ../../.. &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success 'submodule add with // in path' '
 	echo "refs/heads/master" >expect &&
-	>empty &&
 
 	(
 		cd addtest &&
@@ -252,12 +247,11 @@ test_expect_success 'submodule add with // in path' '
 	inspect addtest/slashslashsubmod/frotz ../../.. &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success 'submodule add with /.. in path' '
 	echo "refs/heads/master" >expect &&
-	>empty &&
 
 	(
 		cd addtest &&
@@ -269,12 +263,11 @@ test_expect_success 'submodule add with /.. in path' '
 	inspect addtest/realsubmod ../.. &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success 'submodule add with ./, /.. and // in path' '
 	echo "refs/heads/master" >expect &&
-	>empty &&
 
 	(
 		cd addtest &&
@@ -286,7 +279,7 @@ test_expect_success 'submodule add with ./, /.. and // in path' '
 	inspect addtest/realsubmod2 ../.. &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success !CYGWIN 'submodule add with \\ in path' '
@@ -305,7 +298,6 @@ test_expect_success !CYGWIN 'submodule add with \\ in path' '
 
 test_expect_success 'submodule add in subdirectory' '
 	echo "refs/heads/master" >expect &&
-	>empty &&
 
 	mkdir addtest/sub &&
 	(
@@ -318,7 +310,7 @@ test_expect_success 'submodule add in subdirectory' '
 	inspect addtest/realsubmod3 ../.. &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
-	test_cmp empty untracked
+	test_must_be_empty untracked
 '
 
 test_expect_success 'submodule add in subdirectory with relative path should fail' '
@@ -501,8 +493,6 @@ test_expect_success 'checkout superproject with subproject already present' '
 '
 
 test_expect_success 'apply submodule diff' '
-	>empty &&
-
 	git branch second &&
 	(
 		cd init &&
@@ -517,7 +507,7 @@ test_expect_success 'apply submodule diff' '
 	git apply --index P.diff &&
 
 	git diff --cached master >staged &&
-	test_cmp empty staged
+	test_must_be_empty staged
 '
 
 test_expect_success 'update --init' '