Merge branch 'en/merge-recursive-tests'

Clean up tests in t6xxx series about 'merge' command.

* en/merge-recursive-tests:
  t6036: prefer test_when_finished to manual cleanup in following test
  t6036, t6042: prefer test_cmp to sequences of test
  t6036, t6042: prefer test_path_is_file, test_path_is_missing
  t6036, t6042: use test_line_count instead of wc -l
  t6036, t6042: use test_create_repo to keep tests independent

Author: gitster