Since commit 55218("checkout: do not lose staged removal"), in
cmd_add/cmd_update, "git checkout <commit>" following
"git clone -n" may fail if <commit> is different from HEAD.
So Use "git checkout -f <commit>" to fix this.
Signed-off-by: Ping Yin <pkufranky@gmail.com> Signed-off-by: Shawn O. Pearce <spearce@spearce.org>