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

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>