Merge branch 'tk/subtree-merge-not-ff-only' into jch

When "git subtree" wants to create a merge, it used "git merge" and
let it be affected by end-user's "merge.ff" configuration, which
has been corrected.

* tk/subtree-merge-not-ff-only:
  subtree: force merge commit

contrib/subtree/git-subtree.sh

@@ -975,10 +975,10 @@ cmd_merge () {
 
 	if test -n "$arg_addmerge_message"
 	then
-		git merge -Xsubtree="$arg_prefix" \
+		git merge --no-ff -Xsubtree="$arg_prefix" \
 			--message="$arg_addmerge_message" "$rev"
 	else
-		git merge -Xsubtree="$arg_prefix" $rev
+		git merge --no-ff -Xsubtree="$arg_prefix" $rev
 	fi
 }