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>

1 files changed, 2 insertions, 4 deletions

diff --git a/t/t9903-bash-prompt.sh b/t/t9903-bash-prompt.sh
index c3b89ae..056bf1d 100755
--- a/t/t9903-bash-prompt.sh
+++ b/t/t9903-bash-prompt.sh
@@ -516,10 +516,9 @@ test_expect_success 'prompt - format string starting with dash' '
 
 test_expect_success 'prompt - pc mode' '
 	printf "BEFORE: (\${__git_ps1_branch_name}):AFTER\\nmaster" >expected &&
-	printf "" >expected_output &&
 	(
 		__git_ps1 "BEFORE:" ":AFTER" >"$actual" &&
-		test_cmp expected_output "$actual" &&
+		test_must_be_empty "$actual" &&
 		printf "%s\\n%s" "$PS1" "${__git_ps1_branch_name}" >"$actual"
 	) &&
 	test_cmp expected "$actual"
@@ -715,13 +714,12 @@ test_expect_success 'prompt - hide if pwd ignored - env var set, config disabled
 '
 
 test_expect_success 'prompt - hide if pwd ignored - env var set, config unset' '
-	printf "" >expected &&
 	(
 		cd ignored_dir &&
 		GIT_PS1_HIDE_IF_PWD_IGNORED=y &&
 		__git_ps1 >"$actual"
 	) &&
-	test_cmp expected "$actual"
+	test_must_be_empty "$actual"
 '
 
 test_expect_success 'prompt - hide if pwd ignored - env var set, config unset, pc mode' '