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 d8f4790
("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>

builtin/log.c

@@ -637,6 +637,10 @@ static void log_setup_revisions_tweak(struct rev_info *rev,
 	/* Turn --cc/-c into -p --cc/-c when -p was not given */
 	if (!rev->diffopt.output_format && rev->combine_merges)
 		rev->diffopt.output_format = DIFF_FORMAT_PATCH;
+
+	/* Turn -m on when --cc/-c was given */
+	if (rev->combine_merges)
+		rev->ignore_merges = 0;
 }
 
 int cmd_log(int argc, const char **argv, const char *prefix)