cmd/gopherbot: remember which issues are discovered to be gone
cmd/gopherbot: remember which issues are discovered to be gone

Issues that are gone don't come back. When gopherbot encounters such
issues, it might as well remember it for the next iteration.

This change makes it so that repeating actions without effect
happen once per gopherbot startup, rather than on each iteration.
This is to reduce needless GitHub API quota consumption.

Also update the initial deletedIssues value with newly gone issues.

For golang/go#28320.
Updates golang/go#40640.

Change-Id: I36f3cb780c96ac407482d6c40b4c3e935286e10b
Reviewed-on: https://go-review.googlesource.com/c/build/+/397634
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build