`Process#waitpid` returns `Process::Status`, which holds a 16-bit value.

* The higher-order 8 bits hold the exit() code (`exitstatus`).
* The lower-order bits holds whether the process was terminated.

Previously if the process didn't exit normally, `Gitlab::Popen#popen`
would return a status of `nil` since `exitstatus` would be `nil`.

This isn't informative because we want to know what signal killed the
process. We can get this by calling `Process::Status.to_i`.

Relates to #351155

Changelog: fixed