commit-tree: allow generic object name for the tree as well.

We use get_sha1() for -p (parent) objects, but still used
get_sha1_hex() for the tree.  Just to be consistent, allow
extended SHA1 expression for the tree object name.

Note that this is not to encourage funky things like this:

	git-commit-tree HEAD^{tree} -p HEAD

Signed-off-by: Junio C Hamano <junkio@cox.net>

commit-tree.c

@@ -92,7 +92,7 @@ int main(int argc, char **argv)
 
 	git_config(git_default_config);
 
-	if (argc < 2 || get_sha1_hex(argv[1], tree_sha1) < 0)
+	if (argc < 2 || get_sha1(argv[1], tree_sha1) < 0)
 		usage(commit_tree_usage);
 
 	check_valid(tree_sha1, tree_type);