fix: prevent underflows while trying to abs() very large numbers.

For example, "@{-9223372036854775808}" could trigger a panic previously,
but now it will do the right thing.

gix-revision/src/spec/parse/function.rs

@@ -422,7 +422,7 @@ where
                 if n < 0 {
                     if name.is_empty() {
                         delegate
-                            .nth_checked_out_branch(n.abs().try_into().expect("non-negative isize fits usize"))
+                            .nth_checked_out_branch(n.unsigned_abs())
                             .ok_or(Error::Delegate)?;
                     } else {
                         return Err(Error::RefnameNeedsPositiveReflogEntries { nav: nav.into() });

gix-revision/tests/spec/parse/anchor/at_symbol.rs

@@ -10,6 +10,13 @@ fn braces_must_be_closed() {
     }
 }
 
+#[test]
+#[cfg_attr(target_pointer_width = "32", ignore = "Only works this way on 64 bit systems")]
+fn fuzzed() {
+    let rec = parse("@{-9223372036854775808}");
+    assert_eq!(rec.nth_checked_out_branch, [Some(9223372036854775808), None]);
+}
+
 #[test]
 fn reflog_by_entry_for_current_branch() {
     for (spec, expected_entry) in [("@{0}", 0), ("@{42}", 42), ("@{00100}", 100)] {