sha1_file: reorder code in prepare_packed_git_one()

sha1_file: reorder code in prepare_packed_git_one()

The current loop does

	while (...) {
		if (it is not an .idx file)
			continue;
		process .idx file;
	}

and is reordered to

	while (...) {
		if (it is an .idx file) {
			process .idx file;
		}
	}

This makes it easier to add new extension file processing.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>