- improve output of assertEquals failures on multi-line strings
- avoid false negative failures due to trivial differences in diff hunk headers

  When a hunk header refers to a single line in the "from"
  file and/or the "to" file, e.g.

    @@ -37,37 +41,43 @@
    @@ -37,39 +41,41 @@
    @@ -37,37 +41,41 @@

  some systems will avoid repeating the line number:

    @@ -37 +41,43 @@
    @@ -37,39 +41 @@
    @@ -37 +41 @@

  so we need to canonise the output to avoid false negative
  test failures.
- tests.test_difffiles: added TODO to __canonise_diff.