diff-files: move misplaced cleanup label

Commit 0139c58ab9 (revisions API users: add "goto cleanup" for release_revisions(), 2022-04-13) converted an early return in cmd_diff_files() into a goto. But it put the cleanup label too early: if read_cache_preload() returns an error, we'll set result to "-1", but then jump to calling run_diff_files(), overwriting our result. We should jump past the call to run_diff_files(). Likewise, we should go past diff_result_code(), which is expecting to see a code from an actual diff, not a negative error code. In practice, I suspect this bug cannot actually be triggered, because read_cache_preload() does not seem to ever return an error. Its return value (eventually) comes from do_read_index(), which gives the number of cache entries found, and calls die() on error. Still, it makes sense to fix the inadvertent change from 0139c58ab9 first, and we can look into the overall error handling of read_cache() separately (which is present in many other callsites). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>