gpg-interface: set trust level of missing key to "undefined"

In check_signature(), we initialize the trust_level field to "-1", with the idea that if gpg does not return a trust level at all (if there is no signature, or if the signature is made by an unknown key), we'll use that value. But this has two problems: 1. Since the field is an enum, it's up to the compiler to decide what underlying storage to use, and it only has to fit the values we've declared. So we may not be able to store "-1" at all. And indeed, on my system (linux with gcc), the resulting enum is an unsigned 32-bit value, and -1 becomes 4294967295. The difference may seem academic (and you even get "-1" if you pass it to printf("%d")), but it means that code like this: status |= sigc->trust_level < configured_min_trust_level; does not necessarily behave as expected. This turns out not to be a bug in practice, though, because we keep the "-1" only when gpg did not report a signature from a known key, in which case the line above: status |= sigc->result != 'G'; would always set status to non-zero anyway. So only a 'G' signature with no parsed trust level would cause a problem, which doesn't seem likely to trigger (outside of unexpected gpg behavior). 2. When using the "%GT" format placeholder, we pass the value to gpg_trust_level_to_str(), which complains that the value is out of range with a BUG(). This behavior was introduced by 803978da49 (gpg-interface: add function for converting trust level to string, 2022-07-11). Before that, we just did a switch() on the enum, and anything that wasn't matched would end up as the empty string. Curiously, solving this by naively doing: if (level < 0) return ""; in that function isn't sufficient. Because of (1) above, the compiler can (and does in my case) actually remove that conditional as dead code! We can solve both by representing this state as an enum value. We could do this by adding a new "unknown" value. But this really seems to match the existing "undefined" level well. GPG describes this as "Not enough information for calculation". We have tests in t7510 that trigger this case (verifying a signature from a key that we don't have, and then checking various %G placeholders), but they didn't notice the BUG() because we didn't look at %GT for that case! Let's make sure we check all %G placeholders for each case in the formatting tests. The interesting ones here are "show unknown signature with custom format" and "show lack of signature with custom format", both of which would BUG() before, and now turn %GT into "undefined". Prior to 803978da49 they would have turned it into the empty string, but I think saying "undefined" consistently is a reasonable outcome, and probably makes life easier for anyone parsing the output (and any such parser had to be ready to see "undefined" already). The other modified tests produce the same output before and after this patch, but now we're consistently checking both %G? and %GT in all of them. Signed-off-by: Jeff King <peff@peff.net> Reported-by: Rolf Eike Beer <eb@emlix.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>