subtree: force merge commit

When `merge.ff` is set to `only` in .gitconfig, `git subtree pull` will
fail with error `fatal: Not possible to fast-forward, aborting.`, but
the command does want to make merges in these places. Add `--no-ff`
argument to `git merge` to enforce this behaviour.

Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Koutcher <thomas.koutcher@online.fr>

contrib/subtree/git-subtree.sh

@@ -976,10 +976,10 @@ 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
 }