[PATCH 03/16] t2018: use test_must_fail for failing git commands

[Denton Liu <liu.denton@gmail.com>]

Before, when we expected `git diff` to fail, we negated its status with
`!`. However, if git ever exits unexpectedly, say due to a segfault, we
would not be able to tell the difference between that and a controlled
failure. Use `test_must_fail git diff` instead so that if an unepxected
failure occurs, we can catch it.

We had some instances of `test_must_fail test_dirty_{un,}mergable`.
However, `test_must_fail` should only be used on git commands. Teach
test_dirty_{un,}mergable() to accept `!` as a potential first argument
which will run the git command without test_must_fail(). This prevents
the double-negation where we were effectively running
`test_must_fail test_must_fail git diff ...`.

While we're at it, remove redirections to /dev/null since output is
silenced when running without `-v` and debugging output is useful with
`-v`, remove redirections to /dev/null as it is not useful.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
---
 t/t2018-checkout-branch.sh | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/t/t2018-checkout-branch.sh b/t/t2018-checkout-branch.sh
index 79b16e4677..e6b852939c 100755
--- a/t/t2018-checkout-branch.sh
+++ b/t/t2018-checkout-branch.sh
@@ -33,7 +33,12 @@ do_checkout () {
 }
 
 test_dirty_unmergeable () {
-	! git diff --exit-code >/dev/null
+	should_fail="test_expect_code 1" &&
+	if test "x$1" = "x!"
+	then
+		should_fail=
+	fi &&
+	$should_fail git diff --exit-code
 }
 
 setup_dirty_unmergeable () {
@@ -41,7 +46,12 @@ setup_dirty_unmergeable () {
 }
 
 test_dirty_mergeable () {
-	! git diff --cached --exit-code >/dev/null
+	should_fail="test_expect_code 1"  &&
+	if test "x$1" = "x!"
+	then
+		should_fail=
+	fi &&
+	$should_fail git diff --cached --exit-code
 }
 
 setup_dirty_mergeable () {
@@ -93,7 +103,7 @@ test_expect_success 'checkout -f -b to a new branch with unmergeable changes dis
 
 	# still dirty and on branch1
 	do_checkout branch2 $HEAD1 "-f -b" &&
-	test_must_fail test_dirty_unmergeable
+	test_dirty_unmergeable !
 '
 
 test_expect_success 'checkout -b to a new branch preserves mergeable changes' '
@@ -111,7 +121,7 @@ test_expect_success 'checkout -f -b to a new branch with mergeable changes disca
 	test_when_finished git reset --hard HEAD &&
 	setup_dirty_mergeable &&
 	do_checkout branch2 $HEAD1 "-f -b" &&
-	test_must_fail test_dirty_mergeable
+	test_dirty_mergeable !
 '
 
 test_expect_success 'checkout -b to an existing branch fails' '
@@ -162,7 +172,7 @@ test_expect_success 'checkout -B to an existing branch with unmergeable changes
 test_expect_success 'checkout -f -B to an existing branch with unmergeable changes discards changes' '
 	# still dirty and on branch1
 	do_checkout branch2 $HEAD1 "-f -B" &&
-	test_must_fail test_dirty_unmergeable
+	test_dirty_unmergeable !
 '
 
 test_expect_success 'checkout -B to an existing branch preserves mergeable changes' '
@@ -179,7 +189,7 @@ test_expect_success 'checkout -f -B to an existing branch with mergeable changes
 
 	setup_dirty_mergeable &&
 	do_checkout branch2 $HEAD1 "-f -B" &&
-	test_must_fail test_dirty_mergeable
+	test_dirty_mergeable !
 '
 
 test_expect_success 'checkout -b <describe>' '
-- 
2.24.1.810.g65a2f617f4