tests: use 'test_must_be_empty' instead of 'test_cmp /dev/null <out>'

Using 'test_must_be_empty' is more idiomatic than 'test_cmp /dev/null
out', and its message on error is perhaps a bit more to the point.

This patch was basically created by running:

  sed -i -e 's%test_cmp /dev/null%test_must_be_empty%' t[0-9]*.sh

with the exception of the change in 'should not fail in an empty repo'
in 't7401-submodule-summary.sh', where it was 'test_cmp output
/dev/null'.

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

t/t3210-pack-refs.sh

@@ -127,7 +127,7 @@ test_expect_success 'explicit pack-refs with dangling packed reference' '
 	git reflog expire --expire=all --all &&
 	git prune --expire=all &&
 	git pack-refs --all 2>result &&
-	test_cmp /dev/null result
+	test_must_be_empty result
 '
 test_expect_success 'delete ref with dangling packed version' '
@@ -139,7 +139,7 @@ test_expect_success 'delete ref with dangling packed version' '
 	git reflog expire --expire=all --all &&
 	git prune --expire=all &&
 	git branch -d lamb 2>result &&
-	test_cmp /dev/null result
+	test_must_be_empty result
 '
 test_expect_success 'delete ref while another dangling packed ref' '
@@ -150,7 +150,7 @@ test_expect_success 'delete ref while another dangling packed ref' '
 	git reflog expire --expire=all --all &&
 	git prune --expire=all &&
 	git branch -d lamb 2>result &&
-	test_cmp /dev/null result
+	test_must_be_empty result
 '
 test_expect_success 'pack ref directly below refs/' '

t/t3308-notes-merge.sh

@@ -183,7 +183,7 @@ test_expect_success 'merge empty notes ref (z => y)' '
 	git notes add -m "foo" &&
 	git notes remove &&
 	git notes >output_notes_z &&
-	test_cmp /dev/null output_notes_z &&
+	test_must_be_empty output_notes_z &&
 	# Do the merge (z => y)
 	git config core.notesRef refs/notes/y &&
 	git notes merge z &&

t/t3310-notes-merge-manual-resolve.sh

@@ -337,7 +337,7 @@ EOF
 	git notes merge --commit &&
 	# No .git/NOTES_MERGE_* files left
 	test_might_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
-	test_cmp /dev/null output &&
+	test_must_be_empty output &&
 	# Merge commit has pre-merge y and pre-merge z as parents
 	test "$(git rev-parse refs/notes/m^1)" = "$(cat pre_merge_y)" &&
 	test "$(git rev-parse refs/notes/m^2)" = "$(cat pre_merge_z)" &&
@@ -399,7 +399,7 @@ test_expect_success 'abort notes merge' '
 	git notes merge --abort &&
 	# No .git/NOTES_MERGE_* files left
 	test_might_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
-	test_cmp /dev/null output &&
+	test_must_be_empty output &&
 	# m has not moved (still == y)
 	test "$(git rev-parse refs/notes/m)" = "$(cat pre_merge_y)" &&
 	# Verify that other notes refs has not changed (w, x, y and z)
@@ -466,7 +466,7 @@ EOF
 	git notes merge --commit &&
 	# No .git/NOTES_MERGE_* files left
 	test_might_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
-	test_cmp /dev/null output &&
+	test_must_be_empty output &&
 	# Merge commit has pre-merge y and pre-merge z as parents
 	test "$(git rev-parse refs/notes/m^1)" = "$(cat pre_merge_y)" &&
 	test "$(git rev-parse refs/notes/m^2)" = "$(cat pre_merge_z)" &&
@@ -555,7 +555,7 @@ test_expect_success 'resolve situation by aborting the notes merge' '
 	git notes merge --abort &&
 	# No .git/NOTES_MERGE_* files left
 	test_might_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
-	test_cmp /dev/null output &&
+	test_must_be_empty output &&
 	# m has not moved (still == w)
 	test "$(git rev-parse refs/notes/m)" = "$(git rev-parse refs/notes/w)" &&
 	# Verify that other notes refs has not changed (w, x, y and z)

t/t4047-diff-dirstat.sh

@@ -940,7 +940,7 @@ test_expect_success 'diff.dirstat=0,lines' '
 test_expect_success '--dirstat=future_param,lines,0 should fail loudly' '
 	test_must_fail git diff --dirstat=future_param,lines,0 HEAD^..HEAD >actual_diff_dirstat 2>actual_error &&
 	test_debug "cat actual_error" &&
-	test_cmp /dev/null actual_diff_dirstat &&
+	test_must_be_empty actual_diff_dirstat &&
 	test_i18ngrep -q "future_param" actual_error &&
 	test_i18ngrep -q "\--dirstat" actual_error
 '
@@ -948,7 +948,7 @@ test_expect_success '--dirstat=future_param,lines,0 should fail loudly' '
 test_expect_success '--dirstat=dummy1,cumulative,2dummy should report both unrecognized parameters' '
 	test_must_fail git diff --dirstat=dummy1,cumulative,2dummy HEAD^..HEAD >actual_diff_dirstat 2>actual_error &&
 	test_debug "cat actual_error" &&
-	test_cmp /dev/null actual_diff_dirstat &&
+	test_must_be_empty actual_diff_dirstat &&
 	test_i18ngrep -q "dummy1" actual_error &&
 	test_i18ngrep -q "2dummy" actual_error &&
 	test_i18ngrep -q "\--dirstat" actual_error

t/t4116-apply-reverse.sh

@@ -42,7 +42,7 @@ test_expect_success 'apply in reverse' '
 	git reset --hard second &&
 	git apply --reverse --binary --index patch &&
 	git diff >diff &&
-	test_cmp /dev/null diff
+	test_must_be_empty diff
 '

t/t5509-fetch-push-namespaces.sh

@@ -44,7 +44,7 @@ test_expect_success 'pushing into a repository using a ref namespace' '
 		test_cmp expected actual &&
 		# Try a namespace with no content
 		git ls-remote "ext::git --namespace=garbage %s ../pushee" >actual &&
-		test_cmp /dev/null actual &&
+		test_must_be_empty actual &&
 		git ls-remote pushee-unnamespaced >actual &&
 		sed -e "s|refs/|refs/namespaces/namespace/refs/|" expected >expected.unnamespaced &&
 		test_cmp expected.unnamespaced actual

t/t5523-push-upstream.sh

@@ -113,7 +113,7 @@ test_expect_success TTY 'quiet push' '
 	ensure_fresh_upstream &&
 	test_terminal git push --quiet --no-progress upstream master 2>&1 | tee output &&
-	test_cmp /dev/null output
+	test_must_be_empty output
 '
 test_done

t/t5541-http-push-smart.sh

@@ -226,7 +226,7 @@ test_expect_success TTY 'push --quiet silences status and progress' '
 	cd "$ROOT_PATH"/test_repo_clone &&
 	test_commit quiet &&
 	test_terminal git push --quiet >output 2>&1 &&
-	test_cmp /dev/null output
+	test_must_be_empty output
 '
 test_expect_success TTY 'push --no-progress silences progress but not status' '

t/t7401-submodule-summary.sh

@@ -300,7 +300,7 @@ test_expect_success 'should not fail in an empty repo' "
 	git init xyzzy &&
 	cd xyzzy &&
 	git submodule summary >output 2>&1 &&
-	test_cmp output /dev/null
+	test_must_be_empty output
 "
 test_done

t/t7810-grep.sh

@@ -483,7 +483,7 @@ test_expect_success 'grep -L -C' '
 test_expect_success 'grep --files-without-match --quiet' '
 	git grep --files-without-match --quiet nonexistent_string >actual &&
-	test_cmp /dev/null actual
+	test_must_be_empty actual
 '
 cat >expected <<EOF