Merge branch 'jc/checkout-m-twoway' into maint

* jc/checkout-m-twoway:
t/t2023-checkout-m.sh: fix use of test_must_fail
checkout_merged(): squelch false warning from some gcc
Test 'checkout -m -- path'
checkout -m: no need to insist on having all 3 stages

diff --cc builtin/checkout.c
Simple merge