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
  ca18205
  :100644 100644 538d21d... 59042d1... M	Makefile
  :100644 100644 410b758... 6c47c3a... M	entry.c
  ca18205
  :100644 100644 30479b4... 59042d1... M	Makefile

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

  $ git-diff-tree -r -m -c --abbrev ca18
  ca18205
  ::100644 100644 100644 538d21d... 30479b4... 59042d1... MM	Makefile

    Combined.

  $ git-diff-tree -r -c --abbrev ca18
  ca18205
  ::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>

diff-tree.c

@@ -6,7 +6,7 @@ static int show_root_diff = 0;
 static int no_commit_id = 0;
 static int verbose_header = 0;
 static int ignore_merges = 1;
-static int combine_merges = 1;
+static int combine_merges = 0;
 static int dense_combined_merges = 0;
 static int read_stdin = 0;
 static int always_show_header = 0;
@@ -248,7 +248,7 @@ int main(int argc, const char **argv)
 			continue;
 		}
 		if (!strcmp(arg, "-m")) {
-			combine_merges = ignore_merges = 0;
+			ignore_merges = 0;
 			continue;
 		}
 		if (!strcmp(arg, "-c")) {

git-whatchanged.sh

@@ -10,7 +10,7 @@ case "$0" in
 	count=
 	test -z "$diff_tree_flags" &&
 		diff_tree_flags=$(git-repo-config --get whatchanged.difftree)
-	diff_tree_default_flags='-M --abbrev' ;;
+	diff_tree_default_flags='-c -M --abbrev' ;;
 *show)
 	count=-n1
 	test -z "$diff_tree_flags" &&