log: show merge commit when --cc is given

log: show merge commit when --cc is given

We defaulted to ignoring merge diffs because long long ago, in a
galaxy far away, we didn't have a great way to show the diffs.  The
whole "--cc" option goes back to January '06 and commit d8f4790e6fe7
("diff-tree --cc: denser combined diff output for a merge commit").
And before that option - so for about 8 months - we had no good way
to show the diffs of merges in a good dense way.  So the whole
"don't show diffs for merges by default" actually made a lot of
sense originally, because our merge diffs were not very useful.

And this was carried forward to this day.  "git log --cc" still
ignores merge commits, and you need to say "git log -m --cc" to view
a sensible rendition of merge and non-merge commits, even with the
previous change to make "--cc" imply "-p".

Teach "git log" that "--cc" means the user wants to see interesting
changes in merge commits by turning "-m" on.

Signed-off-by: Junio C Hamano <gitster@pobox.com>