Fix edge cases regarding comment IDs in issue notification email code

Splitting off from #1560012: Port per-user issue notification email functionality to D7 ...

There are a few TODO comments in the code about edge cases regarding comment IDs. In theory, you could programmatically edit an issue multiple times in the same page load, which should generate separate notifications. Mostly the code is now written to handle this, but there are a couple places where we're not as careful as we could be. Pretty minor, since it works great in the normal cases, but I wanted to open an issue so this doesn't get lost.