peel_onion: do not assume length of x_type globals

When we are parsing "rev^{foo}", we check "foo" against the various global type strings, like "commit_type", "tree_type", etc. This is nicely abstracted, but then we destroy the abstraction completely by using magic numbers that must match the length of the type strings. We could avoid these magic numbers by using skip_prefix. But taking a step back, we can realize that using the "commit_type" global is not really buying us anything. It is not ever going to change from being "commit" without causing severe breakage to existing uses. And even if it did change for some crazy reason, we would want to evaluate its effects on the "rev^{}" syntax, anyway. Let's just switch these to using a custom string literal, as we do for "rev^{object}". The resulting code is more robust to changes in the type strings, and is more readable. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>