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"
In investigating #7947 it has become clear that the storage component of go-git repositories needs closing.
This PR adds this Close function and adds the Close functions as necessary.
In TransferOwnership the ctx.Repo.GitRepo is closed if it is open to help prevent the risk of multiple open files.
Fixes #7947
