peel_onion: disambiguate to favor tree-ish when we know we want a tree-ish

The function already knows when interpreting $foo^{commit} to tell the underlying get_sha1_1() to expect a commit-ish while evaluating $foo. Teach it to do the same when asked for $foo^{tree}; we are expecting a tree-ish and $foo should be disambiguated in favor of a tree-ish, discarding a possible ambiguous match with a blob object. Signed-off-by: Junio C Hamano <gitster@pobox.com>