Email Threading (finally fixed)

[Matthew Pickering]

Hi all,

If anyone received emails from gitlab you may have noticed that the
comments to issues ended up in different threads to the original issue
description.

This was because of a custom patch on our gitlab instance which I
think I have now fixed.

For example issue #20192 now has the correct threading, the original
issue and comment now appear in the same email thread (at least in my
client).

Cheers,

Matt