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