Merge branch 'en/merge-tests'

Updates to "git merge" tests, in preparation for a new merge
strategy backend.

* en/merge-tests:
  t6425: be more flexible with rename/delete conflict messages
  t642[23]: be more flexible for add/add conflicts involving pair renames
  t6422, t6426: be more flexible for add/add conflicts involving renames
  t6423: add an explanation about why one of the tests does not pass
  t6416, t6423: clarify some comments and fix some typos
  t6422: fix multiple errors with the mod6 test expectations
  t6423: fix test setup for a couple tests
  t6416, t6422: fix incorrect untracked file count
  t6422: fix bad check against missing file
  t6418: tighten delete/normalize conflict testcase
  Collect merge-related tests to t64xx

diff --cc t/t6406-merge-attr.sh
Simple merge

diff --cc t/t6437-submodule-merge.sh
Simple merge