git subtree push does not support --squash, as previously illustrated in
6ccc71a9 (contrib/subtree: there's no push --squash, 2015-05-07)
Signed-off-by: Danny Lin <danny0838@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Specify <message> as the commit message for the merge commit.
-OPTIONS FOR add, merge, push, pull
-----------------------------------
+OPTIONS FOR add, merge, and pull
+--------------------------------
--squash::
This option is only valid for add, merge, and pull
commands.