Merge branch 'en/directory-renames-nothanks'

Merge branch 'en/directory-renames-nothanks'

Recent addition of "directory rename" heuristics to the
merge-recursive backend makes the command susceptible to false
positives and false negatives.  In the context of "git am -3",
which does not know about surrounding unmodified paths and thus
cannot inform the merge machinery about the full trees involved,
this risk is particularly severe.  As such, the heuristic is
disabled for "git am -3" to keep the machinery "more stupid but
predictable".

* en/directory-renames-nothanks:
  am: avoid directory rename detection when calling recursive merge machinery
  merge-recursive: add ability to turn off directory rename detection
  t3401: add another directory rename testcase for rebase and am