Teach 'git merge' that some merge strategies no longer exist

Teach 'git merge' that some merge strategies no longer exist

'recur' co-existed with 'recursive' when rewriting it in C, but it no
longer available.  'stupid' was also recently removed.

"git merge -s confused origin" still includes them in the list of
available merge strategies.

[jc: this is a squash of two micropatches]

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>