diff-tree: do not default to -c

diff-tree: do not default to -c

Marco says it breaks qgit.  This makes the flags a bit more
orthogonal.

  $ git-diff-tree -r --abbrev ca18

    No output from this command because you asked to skip merge by
    not having -m there.

  $ git-diff-tree -r -m --abbrev ca18
  ca182053c7710a286d72102f4576cf32e0dafcfb
  :100644 100644 538d21d... 59042d1... M	Makefile
  :100644 100644 410b758... 6c47c3a... M	entry.c
  ca182053c7710a286d72102f4576cf32e0dafcfb
  :100644 100644 30479b4... 59042d1... M	Makefile

    The same "independent sets of diff" as before without -c.

  $ git-diff-tree -r -m -c --abbrev ca18
  ca182053c7710a286d72102f4576cf32e0dafcfb
  ::100644 100644 100644 538d21d... 30479b4... 59042d1... MM	Makefile

    Combined.

  $ git-diff-tree -r -c --abbrev ca18
  ca182053c7710a286d72102f4576cf32e0dafcfb
  ::100644 100644 100644 538d21d... 30479b4... 59042d1... MM	Makefile

    Asking for combined without -m does not make sense, so -c
    implies -m.

We need to supply -c as default to whatchanged, which is a
one-liner.

Signed-off-by: Junio C Hamano <junkio@cox.net>