merge-recursive: improve auto-merging messages with path collisions

merge-recursive: improve auto-merging messages with path collisions

Each individual file involved in a rename could have also been modified
on both sides of history, meaning it may need to have content merges.
If two such files are renamed into the same location, then on top of the
two natural auto-merging messages we also have to two-way merge the
result, giving us messages that look like

  Auto-merging somefile.c (was somecase.c)
  Auto-merging somefile.c (was somefolder.c)
  Auto-merging somefile.c

However, despite the fact that I was the one who put the "(was %s)"
portions into the messages (and just a few months ago), I was still
initially confused when running into a rename/rename(2to1) case and
wondered if somefile.c had been merged three times.  Update this to
instead be:

  Auto-merging version of somefile.c from somecase.c
  Auto-merging version of somefile.c from someportfolio.c
  Auto-merging somefile.c

This is an admittedly long set of messages for a single path, but you
only get all three messages when dealing with the rare case of a
rename/rename(2to1) conflict where both sides of both original files
were also modified, in conflicting ways.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>