Merge branch 'jk/pretty-G-format-fixes' into maint

"%G" (nothing after G) is an invalid pretty format specifier, but
the parser did not notice it as garbage.

* jk/pretty-G-format-fixes:
  move "%G" format test from t7510 to t6006
  pretty: avoid reading past end-of-string with "%G"
  t7510: check %G* pretty-format output
  t7510: test a commit signed by an unknown key
  t7510: use consistent &&-chains in loop
  t7510: stop referring to master in later tests

pretty.c

@@ -1249,6 +1249,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			if (c->signature_check.key)
 				strbuf_addstr(sb, c->signature_check.key);
 			break;
+		default:
+			return 0;
 		}
 		return 2;
 	}