Fixes#14187: mention handling extracted from email notification code
Fixes#14013: add notification for mentions in pull request code comments
Fixes#13450: Not receiving any emails with setting "Only Email on Mention"
Fixes#13683.
The diff snippet that provides context for a code review comment on the pull request timeline page used to be calculated based on the headCommitID. But in 1.13, with PR #13448, this changed to the commitID from the blame for the commented line, which seems to cause these incorrect review comment diff snippets.
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>