diff: support ^! for merges

diff: support ^! for merges

revision.c::handle_revision_arg_1() resolves <rev>^! by first adding the
negated parents and then <rev> itself.  builtin_diff_combined() expects
the first tree to be the merge and the remaining ones to be the parents,
though.  This mismatch results in bogus diff output.

Remember the first tree that doesn't belong to a parent and use it
instead of blindly picking the first one.  This makes "git diff <rev>^!"
consistent with "git show <rev>^!".

Reported-by: Tim Jaacks <tim.jaacks@garz-fricke.com>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>