Add fs::MTime newtype to encourage != instead of > (#20830)

See ["mtime comparison considered
harmful"](https://apenwarr.ca/log/20181113) for details of why
comparators other than equality/inequality should not be used with
mtime.

Release Notes:

- N/A

crates/copilot/src/copilot.rs

@@ -1231,7 +1231,7 @@ mod tests {
 
         fn disk_state(&self) -> language::DiskState {
             language::DiskState::Present {
-                mtime: std::time::UNIX_EPOCH,
+                mtime: ::fs::MTime::from_seconds_and_nanos(100, 42),
             }
         }