[Git] recursive merge on 'master' severely broken?

[Junio C Hamano <gitster@pobox.com>]

It appears that a topic recently graduated to 'master' introduces a
severe regression to "git merge" when it merges a side branch that
renames paths while the trunk has further updates.

The symptom can easily be seen by trying to recreate the merge I
made at the tip of 'pu' 29dea678 ("Merge branch
'sb/filenames-with-dashes' into pu", 2018-04-11) that I'll be.
pushing out shortly.  The side branch renames a file exec_cmd.h to
exec-cmd.h (an underscore changed to a dash) without changing any
contents, while the branch being merged to has made some changes to
the contents while keeping the original pathname.

A clean automerged result should leave the identical contents from
HEAD:exec_cmd.h in :exec-cmd.h in the index, which is what happens
when using Git v2.17.0 proper, but with today's master', there are
content changes that cannot be explained--the merge is simply broken
and worse yet, the command pretends that everything went well and
merged cleanly in that path.  Overly clever tool that behaves in a
buggy and unexplainable way is bad enough, doing so silently is
unexcusable.

I suspect that the culprit is the merge e4bb62fa ("Merge branch
'en/rename-directory-detection'", 2018-04-10), and I am planning to
revert it out of 'master' as soon as possible, but it is already
buried deep in other topics, so I may not be able to get to it until
later this week.  Sorry about that.