Merge branch 'dl/t5520-cleanup'

Test cleanup.

* dl/t5520-cleanup:
  t5520: replace `! git` with `test_must_fail git`
  t5520: remove redundant lines in test cases
  t5520: replace $(cat ...) comparison with test_cmp
  t5520: don't put git in upstream of pipe
  t5520: test single-line files by git with test_cmp
  t5520: use test_cmp_rev where possible
  t5520: replace test -{n,z} with test-lib functions
  t5520: use test_line_count where possible
  t5520: remove spaces after redirect operator
  t5520: replace test -f with test-lib functions
  t5520: let sed open its own input
  t5520: use sq for test case names
  t5520: improve test style
  t: teach test_cmp_rev to accept ! for not-equals
  t0000: test multiple local assignment

diff --cc t/t0000-basic.sh
Simple merge

diff --cc t/t2400-worktree-add.sh
Simple merge

diff --cc t/t3430-rebase-merges.sh
Simple merge