Merge branch 'dl/test-must-fail-fixes-2'

Test updates. * dl/test-must-fail-fixes-2: t4124: only mark git command with test_must_fail t3507: use test_path_is_missing() t3507: fix indentation t3504: do check for conflict marker after failed cherry-pick t3419: stop losing return code of git command t3415: increase granularity of test_auto_{fixup,squash}() t3415: stop losing return codes of git commands t3310: extract common notes_merge_files_gone() t3030: use test_path_is_missing() t2018: replace "sha" with "oid" t2018: don't lose return code of git commands t2018: teach do_checkout() to accept `!` arg t2018: be more discerning when checking for expected exit codes t2018: improve style of if-statement t2018: add space between function name and () t2018: remove trailing space from test description