test: use test_must_be_empty F instead of test -z $(cat F)

Use test_must_be_empty instead of reading the file and comparing its
contents to an empty string.  That's more efficient, as the function
only needs built-in meta-data only check in the usual case, and provides
nicer debug output otherwise.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1309-early-config.sh

@@ -29,7 +29,7 @@ test_expect_success 'ceiling' '
 		cd sub &&
 		test-tool config read_early_config early.config
 	) >output &&
-	test -z "$(cat output)"
+	test_must_be_empty output
 '
 
 test_expect_success 'ceiling #2' '

t/t1506-rev-parse-diagnosis.sh

@@ -138,10 +138,10 @@ test_expect_success 'incorrect file in :path and :N:path' '
 
 test_expect_success 'invalid @{n} reference' '
 	test_must_fail git rev-parse master@{99999} >output 2>error &&
-	test -z "$(cat output)" &&
+	test_must_be_empty output &&
 	grep "fatal: Log for [^ ]* only has [0-9][0-9]* entries." error  &&
 	test_must_fail git rev-parse --verify master@{99999} >output 2>error &&
-	test -z "$(cat output)" &&
+	test_must_be_empty output &&
 	grep "fatal: Log for [^ ]* only has [0-9][0-9]* entries." error
 '
 
@@ -155,13 +155,13 @@ test_expect_success 'relative path not found' '
 
 test_expect_success 'relative path outside worktree' '
 	test_must_fail git rev-parse HEAD:../file.txt >output 2>error &&
-	test -z "$(cat output)" &&
+	test_must_be_empty output &&
 	test_i18ngrep "outside repository" error
 '
 
 test_expect_success 'relative path when cwd is outside worktree' '
 	test_must_fail git --git-dir=.git --work-tree=subdir rev-parse HEAD:./file.txt >output 2>error &&
-	test -z "$(cat output)" &&
+	test_must_be_empty output &&
 	grep "relative path syntax can.t be used outside working tree." error
 '
 

t/t6019-rev-list-ancestry-path.sh

@@ -143,14 +143,14 @@ test_expect_success 'setup criss-cross' '
 test_expect_success 'criss-cross: rev-list --ancestry-path cb..bc' '
 	(cd criss-cross &&
 	 git rev-list --ancestry-path xcb..xbc > actual &&
-	 test -z "$(cat actual)")
+	 test_must_be_empty actual)
 '
 
 # no commits in repository descend from cb
 test_expect_success 'criss-cross: rev-list --ancestry-path --all ^cb' '
 	(cd criss-cross &&
 	 git rev-list --ancestry-path --all ^xcb > actual &&
-	 test -z "$(cat actual)")
+	 test_must_be_empty actual)
 '
 
 test_done