t: prepare `test_must_fail ()` for `set -e`

The helper function `test_must_fail ()` executes a specific Git command
that may or may not fail in a specific way. This is done by executing
the command in question and then comparing its exit code against a set
of conditions.

This works, but once we run our test suite with `set -e` we may bail out
of `test_must_fail ()` early in case the command actually fails, even
though we expect it to fail. Prepare for this change by handling the
failed case with `||`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -1195,8 +1195,9 @@ test_must_fail () {
 		echo >&7 "test_must_fail: only 'git' is allowed: $*"
 		return 1
 	fi
-	"$@" 2>&7
-	exit_code=$?
+
+	exit_code=0; "$@" 2>&7 || exit_code=$?
+
 	if test $exit_code -eq 0 && ! list_contains "$_test_ok" success
 	then
 		echo >&4 "test_must_fail: command succeeded: $*"