subtree: be stricter about validating flags

subtree: be stricter about validating flags

Don't silently ignore a flag that's invalid for a given subcommand.  The
user expected it to do something; we should tell the user that they are
mistaken, instead of surprising the user.

It could be argued that this change might break existing users.  I'd
argue that those existing users are already broken, and they just don't
know it.  Let them know that they're broken.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>