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>

sha1_name.c

@@ -654,6 +654,8 @@ static int peel_onion(const char *name, int len, unsigned char *sha1)
 
 	if (expected_type == OBJ_COMMIT)
 		lookup_flags = GET_SHA1_COMMITTISH;
+	else if (expected_type == OBJ_TREE)
+		lookup_flags = GET_SHA1_TREEISH;
 
 	if (get_sha1_1(name, sp - name - 2, outer, lookup_flags))
 		return -1;